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.
Instead of handling resumed key events in a deep stack, buffer them
until the next main loop iteration. New resume events that may be emitted
during handling of old resumes are prepended to that buffer, i.e. take
precedence over events that happen deeper into the buffer/event stack.
Logical replay order is thus preserved.
