NaN boxing: use uintptr_t for bit manipulations

fix some warnings emitted by 'ruby -w'

- File.exists? is deprecated since 2.1.0
- tasks/mruby_build_commands.rake:268: ambiguous argument

More kernel testing

return-value of mrb_run is invalid in top-level-scope

add String#partition and String#rpartition.

More tests for Class behavior

add tests for Array methods

The return-value of mrb_run in top-level-scope
should be the evaluated value at last.

Merge branch 'singletonfix' of https://github.com/carsonmcdonald/mruby into carsonmcdonald-singletonfix

Change :vattr= to :cattr= in respond_to? test

fix #1617

class/module body to honor whether value is required or not; reduce a few instructions for normal cases