Faster::OpenStruct

Up to 40 (!) times more memory efficient version of OpenStruct

Differences from Ruby MRI OpenStruct:

1. Doesn't dup passed initialization hash (NOTE: only reference to hash is stored)

2. Doesn't convert hash keys to symbols (by default string keys are used, with fallback to symbol keys)

3. Creates methods on the fly on OpenStruct class, instead of singleton class. Uses module_eval with string to avoid holding scope references for every method.

4. Refactored, crud clean, spec covered :)