remove unnecessary CHAR_BIT != 8 check

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.

Use `mrb_bool` as return type of `mrb_const_defined_at`.

Inherit `$mrbtest_verbose` flag in core tests.