Useful when we have pointer to char inside string, but need to return char
index. (E.g. str.find()).

Squashed commit of the following:

commit 99dc21b67a895dc10d3c846bc158d27c839cee48
Author: Chris Angelico <rosuav@gmail.com>
Date:   Thu Jun 12 02:18:54 2014 +1000

    Optimize as per TODO (thanks Damien!)

commit 5bf0153ecad8348443058d449d74504fc458fe51
Author: Chris Angelico <rosuav@gmail.com>
Date:   Tue Jun 10 08:42:06 2014 +1000

    Test a default (= UTF-8) encode and decode

commit c962057ac340832c4fde60896f656a3fe3ad78a9
Merge: e2c9782 195de324
Author: Chris Angelico <rosuav@gmail.com>
Date:   Tue Jun 10 05:23:03 2014 +1000

    Merge branch 'master' into unicode, resolving conflict on py/obj.h

commit e2c9782a65eb57f481d441d40161de427e1940ba
Author: Chris Angelico <rosuav@gmail.com>
Date:   Tue Jun 10 05:05:57 2014 +1000

    More whitespace fixups

commit 086a2a0f57afbc1f731697fd5d3a0cbbb80e5418
Author: Chris Angelico <rosuav@gmail.com>
Date:   Tue Jun 10 05:04:20 2014 +1000

    Properly implement string slicing

commit 0d339a143e2b6442366145e7f3d64aada293eaa0
Author: Chris Angelico <rosuav@gmail.com>
Date:   Tue Jun 10 02:24:11 2014 +1000

    Support slicing in str_index_to_ptr, and fix a bounds error

commit 24371c7267d360e77cf5eabc2e8ce9a73d2ee0da
Author: Chris Angelico <rosuav@gmail.com>
Date:   Tue Jun 10 02:10:22 2014 +1000

    Break out index-to-pointer calculation into a function

commit 616c24ac014c3ca56008428c506034dd1bfff7a8
Author: Chris Angelico <rosuav@gmail.com>
Date:   Tue Jun 10 02:03:11 2014 +1000

    Add tests of string slicing, which currently fail

commit a24d19f676fe8cc21dad512d91b826892e162a5b
Author: Chris Angelico <rosuav@gmail.com>
Date:   Tue Jun 10 01:56:53 2014 +1000

    Change string indexing to not precalculate the charlen, and add test for neg indexing

commit 0bcc7ab89eafb2ae53195e94c9bea42a4e886b64
Author: Chris Angelico <rosuav@gmail.com>
Date:   Sun Jun 8 22:09:17 2014 +1000

    Clean up constant qstr declarations now that charlen isn't needed

commit 5473e1a1dba2124b7b0c207f2964293cfbe80167
Author: Chris Angelico <rosuav@gmail.com>
Date:   Sun Jun 8 07:18:42 2014 +1000

    Remove the charlen field from strings, calculating it when required

commit 5c1658ec71aefbdc88c261ce2e57dc7670cdc6ef
Author: Chris Angelico <rosuav@gmail.com>
Date:   Sun Jun 8 07:11:27 2014 +1000

    Get rid of mp_obj_str_get_data_len() which was used in only one place

commit a019ba968b4e8daf7f3674f63c5cc400e304c509
Author: Chris Angelico <rosuav@gmail.com>
Date:   Sun Jun 8 06:58:26 2014 +1000

    Add a unichar_charlen() function to calculate length-in-characters from length-in-bytes

commit 44b0d5cff846ba487c526ed95be1b3d1cd3d762a
Author: Chris Angelico <rosuav@gmail.com>
Date:   Sun Jun 8 06:32:44 2014 +1000

    Use utf8_get/next_char in building up a string's repr

commit 30d1bad33f7af90f1971987c39864c8fcf3f5c21
Author: Chris Angelico <rosuav@gmail.com>
Date:   Sun Jun 8 06:10:45 2014 +1000

    Make utf8_get_char() and utf8_next_char() actually do what their names say

commit bc990dad9afb8ec112f5e7f7f79d5ab415da0e72
Author: Chris Angelico <rosuav@gmail.com>
Date:   Sun Jun 8 02:10:59 2014 +1000

    Revert "Add PEP 393-flags to strings and stub usage."

    This reverts commit c239f509521d1a0f9563bf9c5de0c4fb9a6a33ba.

commit f9bebb28ad52467f2f2d7a752bb033296b6c2f9b
Author: Chris Angelico <rosuav@gmail.com>
Date:   Sat Jun 7 15:41:48 2014 +1000

    Whitespace fixes

commit 279de0c8eb3cb186914799ccc5ee94ea97f56de4
Author: Chris Angelico <rosuav@gmail.com>
Date:   Sat Jun 7 15:28:35 2014 +1000

    Formatting/layout improvements - introduce macros for UTF-8 byte detection, add braces. No functional changes.

commit f1911f53d56da809c97b07245f5728a419e8fb30
Author: Chris Angelico <rosuav@gmail.com>
Date:   Sat Jun 7 11:56:02 2014 +1000

    Make chr() Unicode-aware

commit f51ad737b48ac04c161197a4012821d50885c4c7
Author: Chris Angelico <rosuav@gmail.com>
Date:   Sat Jun 7 11:44:07 2014 +1000

    Make a string's repr Unicode-aware

commit 01bd68684611585d437982dccdf05b33cbedc630
Author: Chris Angelico <rosuav@gmail.com>
Date:   Sat Jun 7 11:33:43 2014 +1000

    Expand the Unicode tests

commit 7bc91904f899f8012089fc14a06495680a51e590
Author: Chris Angelico <rosuav@gmail.com>
Date:   Sat Jun 7 11:27:30 2014 +1000

    Record byte lengths for byte strings

commit bb132120717cf176dcfb26f87fa309378f76ab5f
Author: Chris Angelico <rosuav@gmail.com>
Date:   Sat Jun 7 11:25:06 2014 +1000

    Make ord() Unicode-aware

commit 03f0cbe9051b62192be97b59f84f63f9216668bf
Author: Chris Angelico <rosuav@gmail.com>
Date:   Sat Jun 7 10:24:35 2014 +1000

    Retain characters as UTF-8 encoded Unicode

commit e924659b85c001916a5ff7f4d1d8b3ebe2bf0c2f
Author: Chris Angelico <rosuav@gmail.com>
Date:   Sat Jun 7 08:37:27 2014 +1000

    Add support for \u and \U escapes, but not \N (with explanatory comment)

commit 231031ac5f0346e4ffcf9c4abec2bd33f566232c
Author: Chris Angelico <rosuav@gmail.com>
Date:   Sat Jun 7 05:09:35 2014 +1000

    Add character length to qstr

commit 6df1b946fb17d8d5df3d91b21cde627c3d4556a8
Author: Chris Angelico <rosuav@gmail.com>
Date:   Fri Jun 6 13:48:36 2014 +1000

    Add test of UTF-8 encoded source file resulting in properly formed string

commit 16429b81a8483cf25865ed11afd81a7d9c253c26
Author: Chris Angelico <rosuav@gmail.com>
Date:   Fri Jun 6 13:44:15 2014 +1000

    Make len(s) return character length (even though creation's still buggy)

commit cd2cf6663cc47831dbc97819ad5c50ad33f939d3
Author: Chris Angelico <rosuav@gmail.com>
Date:   Fri Jun 6 13:15:36 2014 +1000

    HACK - When indexing a qstr, count its charlen. Stupidly inefficient but POC.

    All tests pass now, though string creation is still buggy.

commit 47c234584d3358dfa6b4003d5e7264105d17b8f7
Author: Chris Angelico <rosuav@gmail.com>
Date:   Fri Jun 6 13:15:32 2014 +1000

    objstr: Record character length separately from byte length

    CAUTION: Buggy, may crash stuff - qstr needs equivalent functionality too

commit b0f41c72af27d3b361027146025877b3d7e8785c
Author: Chris Angelico <rosuav@gmail.com>
Date:   Fri Jun 6 05:37:36 2014 +1000

    Beginnings of UTF-8 support - construct strings from that many UTF-8-encoded chars, and subscript bytes the same way

commit 89452be641674601e9bfce86dc71c17c3140a6cf
Author: Chris Angelico <rosuav@gmail.com>
Date:   Fri Jun 6 05:28:47 2014 +1000

    Update comments - now aiming for UTF-8 rather than PEP 393 strings

commit c239f509521d1a0f9563bf9c5de0c4fb9a6a33ba
Author: Chris Angelico <rosuav@gmail.com>
Date:   Wed Jun 4 05:28:12 2014 +1000

    Add PEP 393-flags to strings and stub usage.

    The test suite all passes, but nothing has actually been changed.

Put into misc/ to not complicate life for builds with check disabled.

Such mechanism is important to get stable Python functioning, because Python
function calling is handled with C stack. The idea is to sprinkle
STACK_CHECK() calls in places where there can be C recursion.

TODO: Add more STACK_CHECK()'s.

Use mp_const_none to initialize mapper and map_dict (fix #701)

Also, factor out mp_binary_get_int() function.

Return free/allocated memory on GC heap.

msvc: Enable GC

It sucks to workaround this on uPy side, but upgrading not upgradable
embedded systems sucks even more.

So, allocate one explicitly.

The pointers to the bss section are acquired in init.c()
by inspecting the PE header. Works for msvc and mingw.

These changes were tested with QEMU, and by few people of real hardware.