modify Array#reject!
delete ';' in Enumerable#each_slice
add mrb-file: hash.rb, add test-file: hash.rb
implement Hash#delete_if by Hash#reject!
add Hash#keep_if
implement Hash#keep_if by Hash#select!
fix Hash#keep_if
fix test for Hash#delete_if, Hash#keep_if
fix Hash#keep_if 's comments
add Array#select!, Array#keep_if