cache_memoize() and cache_memoize_iter() didn't work well with type

hints before. Type checkers would infer a list of possible types, none

of which were likely to match the true type that would be returned, and

this forced all callers to either cast() or add a type: ignore

comment.
These now infer the return types based on the callback function

provided. This helps ensure the right type in the majority of cases.

There's a caveat in that the actual cached data may always be of some

other type, but that's already something the caller was responsible for

handling.
To keep things maintainable, these methods, along with

make_cache_key(), now require keyword-only arguments for all optional

flags and settings. For a simple usage of cache_memoize() or

cache_memozie_iter(), nothing changes, as the key and

lookup_callable arguments can still be provided as positional

arguments, but all others are now optional.
We're also taking the time to properly deprecate some old arguments,

which will be removed once keyword-only arguments become enforced.

Unit tests pass in Djblets and Review Board.