Move Hash#values_at to mruby-hash-ext gem.
Move comments from hash.c to hash.rb.
Add more test cases for test coverage.
Remove mrb_hash_lookup() as no one uses it. Use mrb_hash_get instead.
Merge pull request #1237 from monaka/pr-cleanup-hash

Cleanup hash
mirb should handle exception from inspect; close #1238
Merge branch 'master' of github.com:mruby/mruby