Various speed improvements to reach near-parity with RecursiveOpenStruct:
Various speed improvements to reach near-parity with RecursiveOpenStruct:

* don't 'construct' the key.
* lazy-load the class definitions.
* use class_eval instead of instance_eval.
* don't process values that are irrelevant.
* don't dup (may not be wise).

Kuraiou/config_hash