Add information about Kernel#binding

Run tests parallelly for each target.

Increase sleep duration exponentially instead of `Thread.pass`.

Use Fiber#transfer instead in minirake.

Use `Thread.pass` instead of sleeping.

Fix job waiter of minirake.

Show minirake backtrace in verbose mode.

Use temporary file name for `File#mtime` test to avoid conflicts.

Make minirake parallel.

Fix build scripts.

Fix memory leaks in mirb.

This will improve the default performance of mruby; fix #4045

Export mrb_utf8_from_locale() and mrb_locale_from_utf8()

Stop to use freed value

Fix wrong free function