Update document and add :doc task.

Fix memory leaks in String

Refactor how to use RSTR_NOFREE_P macro

Add NOFREE macros

rename mrb_atexit to mrb_state_atexit to clarify a func will be called at the end of state, not process; ref #2211

Append include paths in dependency gems

Add ".s" to source extensions.

Find c++ files in tools/.

Do not export include/ dirs in mrbgems to core build.

Add macro `mrb_static_assert`.

Add a missing space after ","

Fix `Time.at` args spec.

In the current behavior, include/ dirs in mrbgems are set to
core's compiler.include_paths.
And they are never set to mrbgem's include_paths.
It may cause some dangerous issues because it can change
mruby core's macros by mrbgems.

After this fix, include/ in a gem is used in the gem itself only.

Some tools may be written in C++.

It uses ".s" as the source for assembler on some toolchains including GCC.