This patch provides inline versions of the utf8 helper functions for the
case when unicode is disabled (MICROPY_PY_BUILTINS_STR_UNICODE set to 0).
This saves code size.

The unichar_charlen function is also renamed to utf8_charlen to match the
other utf8 helper functions, and the signature of this function is adjusted
for consistency (const char* -> const byte*, mp_uint_t -> size_t).

This patch adds a function utf8_check() to check for a valid UTF-8 encoded
string, and calls it when constructing a str from raw bytes.  The feature
is selectable at compile time via MICROPY_PY_BUILTINS_STR_UNICODE_CHECK and
is enabled if unicode is enabled.  It costs about 110 bytes on Thumb-2, 150
bytes on Xtensa and 170 bytes on x86-64.

There were several different spellings of MicroPython present in comments,
when there should be only one.

- As described in the #1850.
- Add cmdline tests.

This drops the size of unicode_isxdigit from 0x1e + 0x02 filler to
0x14 bytes (so net code reduction of 12 bytes) and will make
unicode_is_xdigit perform slightly faster.

This also pulls out hex_digit from py/lexer.c and makes unichar_hex_digit

Addresses issue #1022.

See discussion in issue #50.

Based on config define.

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

It defines types used by all other headers.

Fixes #691.

We still have that char vs byte dichotomy, but majority of string operations
now use byte.

Blanket wide to all .c and .h files.  Some files originating from ST are
difficult to deal with (license wise) so it was left out of those.

Also merged modpyb.h, modos.h, modstm.h and modtime.h in stmhal/.

Eg, in CPython stdlib, email/header.py has a form-feed character.