remove unnecessary CHAR_BIT != 8 check

mruby uses uint8_t which implies CHAR_BIT == 8.
fix conversion warnings

Those warnings are not enabled by default,
but getting rid of them doesn't hurt.
MSVC: add simple (v)snprintf implementation

_snprintf is not C99's snprintf!

This simple implementation, unlike _snprintf,
always null-terminates and returns the expected
return value (in most cases).
Other C99 behaviors (like format specifiers) require
adding a complete snprintf implementation.
Do we want or need that?

The same applies to vsnprintf (aka _vsnprintf).
change linkage of mrb_str_size to internal

Use RSTRING_LEN to get the length.
Merge pull request #2496 from cremno/remove-unnecessary-char_bit-check

remove unnecessary CHAR_BIT != 8 check
Merge pull request #2497 from cremno/fix-conversion-warnings

fix conversion warnings (in gc.c)
Merge pull request #2501 from cremno/static-mrb_str_size

change linkage of mrb_str_size to internal
Merge pull request #2498 from cremno/msvc-snprintf

MSVC: add simple (v)snprintf implementation