void* is of unknown size

Functionality we provide in builtin io module is fairly minimal. Some
code, including CPython stdlib, depends on more functionality. So, there's
a choice to either implement it in C, or move it _io, and let implement other
functionality in Python. 2nd choice is pursued. This setup matches CPython
too (_io is builtin, io is Python-level).

It needs to be reused for generator functions, too.

Also now returns correct POSIX errno when an IO operation fails.

Addresses issues #516 and #676.

Remove unnecessary bounds check from mp_seq_get_fast_slice_indexes.

At this point, start will be >= 0, so checking if stop < 0 is redundant with
checking if start > stop a few lines later.

This is getting more and more tangled, but that's old news.

As before, instead of pushing constant values on stack again and again, just
pass around pointer to a structure.

See PR #618.

Conflicts:
	py/emitglue.c

Should finish addressing issue #524.

Ports which wants to have it, should define MICROPY_PY_SYS_PLATFORM to a
string value they need.

Benefits: won't crash baremetal targets, will provide Python source location
when not implemented feature used (it will no longer provide C source
location, but just grep for error message).

This reduces stack usage by 16 words (64 bytes) for stmhal/ port.

See issue #640.

?= operator does not do delayed expansion (unlike =).

toolchain fixes to enable cross compatibility

Build patches for FreeBSD (as discussed in the former pull request #666)

  cast error in MP_OBJ_NEW_SMALL_INT(). This is necessary for FreeBSD, where
  st_ino is of different size
- If MP_CLOCKS_PER_SEC is defined on the target host, simply define CLOCK_DIV
  as a fraction, regardless of the value of MP_CLOCKS_PER_SEC.
  FreeBSD uses a non-POSIX compliant value of 128 for CLOCKS_PER_SEC

- Move the includes for alloca() intp mpconfigport.h

  there are special tweaks and paths to be considered. Just provide some
  defaults, in case the values are undefined.
- py-version.sh does not need any bash specific features.
- Use libdl only on Linux for now. FreeBSD provides dl*() calls from libc.

unix: Fix path seperator used depending on OS