_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).

Use Boolean macro in variable.c

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.