Reduce implicit type casts.
remove size_t cast in mrb_assert() since ((size_t)n <= SIZE_MAX) is always true
avoid Dir.exist? that is not available in Ruby1.8

Conflicts:

	tasks/mruby_build.rake
use File#directory? instead of File#exist?; ref c2d8b0

Conflicts:

	tasks/mruby_build.rake