Instead of indexing `Key` objects that have multiple names by each
individual name, index by the set of names. This reduces the size of the
default key dictionary by a factor of between 2 and 3, and as result
also reduces reallocations/defragmentation.
Instead of instantiating all module/extension keys by default, append
them to the `maybe_key`-generator list.
- Add tests for ModifierKey chaining
- Rewrite ModifierKey.__call__ for correctness and readability
- __call__ now maintains handlers and meta, like clone()
