WIP: Simplifying Provider's interface
Have Provider#[] return PropertyValue
Fix Provider#keys
Simplify Provider even more
Make Provider easier to subclass
Keep tweaking Providers subclassing interface
Make provider specs more complete and legible
Give more responsibility to PropertyValue
Rename PropertyValue Property and fix naming

A Property now consists of a key and a value
Use Properties in and simplify Configuration

By not caching values, not merging Hashes, etc.