Update augmentation libraries proposal.

The main change is that augmentations share a top level and private
namespace with each other and the main library. Also made a few other
minor changes along the way:

*   Now that enums have members, allow them to be augmented.

*   Compile-time error if a non-`late` augmenting instance variable
    calls the initializer for a `late` one.

Author: munificent