Implement Hash#to_h
Implement Struct#to_h
Implement NilClass#to_h
Remove ENV.to_h(Rubinius::EnvironmentVariables#to_h) from 1.9

ENV.to_h is a new feature of Ruby2.0.
Implement OpenStruct#to_h

Ported from [MRI](https://github.com/ruby/ruby/blob/896af106070c9da72f45bf16b47c6641a6fdbc62/lib/ostruct.rb#L105-L116)
Merge pull request #2454 from kachick/2.0-to_h

{Hash,Struct,NilClass,ENV,OpenStruct}.#to_h for 2.0