change behavior of mrb_sym2str
Remove 'mrb_state' field from 'kh_xxx_t' structure.

'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.
Merge pull request #1614 from h2so5/change-sym2str

change behavior of mrb_sym2str
Merge pull request #1615 from crimsonwoods/remove_strong_coupling_by_khash

Remove 'mrb_state' field from 'kh_xxx_t' structure.