Refactor key meta

[xs5871]

As promised in #988:

Coming soon: getting rid of key.meta, aka: "we already use classes for key types, but only sometimes, and we hide it behind a layer of indirection".

All key.meta classes and attributes have been refactored into flat Key classes, plus some cleanup and code simplification.
Anecdotal performance improvement: my personal config uses ~2kB less RAM with these changes.

[regicidalplutophage]

It even takes up less memory, nice!