Simplify augmentations. (#4357)

Simplify augmentations.

- Remove references to macros.
- Don't allow augmentations to wrap or replace code. Remove support for `augmented` expressions. Disallow an augmentation from providing a body to a declaration that already has one.
- Remove support for augmenting variables.
- Simplify constructor augmentations: no concatenating initializers or merging initializers from one augmentation and a body from another.
- Remove support for augmenting typedefs.
- Remove support for augmenting redirecting constructors.
- Allow a function augmentation to have an `external` body.

There are still more changes I'd like to make. In particular, the grammar for members seems pretty hairy and I suspect could be refactored to be simpler, but I'm not sure if that will cause problems for other people who rely on the existing grammar rules.

Author: munificent