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

verify the argument of String#=~

Remove 'mrb_state' field from 'kh_xxx_t' structure.

change behavior of mrb_sym2str

'kh_xxx_t' requires 'mrb_state' to allocate, free, and compute hash value.
But 'mrb_state' should not be held by 'kh_xxx_t' and 'mrb_state' should be
supplied from outside.

add some tests for case statements