fix linkage of range_beg_len and mrb_range_beg_len

MSVC: add simple (v)snprintf implementation

change linkage of mrb_str_size to internal

fix conversion warnings (in gc.c)

remove unnecessary CHAR_BIT != 8 check

Use RSTRING_LEN to get the length.

_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).

Those warnings are not enabled by default,
but getting rid of them doesn't hurt.

Use Boolean macro in variable.c

mruby uses uint8_t which implies CHAR_BIT == 8.

mirb: allocate `history_path` dynamically

Refactor sym_equal() in symbol.c

add Module#module_function

Add field `allocf_ud` to replace current `ud`.

Silence "unused parameter" warnings in C++

This has been done already, but the fixes were accidentally
removed in 249f05e7.

Fix memory leak for test driver

Use MRB_TT_EXCEPTION in exception object.

Move word boxing structs to "mruby/boxing_word.h".

Print backtrace of crashed test in verbose mode.

Since some use it as `mrb_state` associated user data.

Check mrbgem.rake instead of directory to ignore empty directories in full-core gembox.