Expose `has` on memoized methods

[rektide]

I'd like to see a more general solution that also lets us iterate through all inputs that have been memoized.

[rektide]

Rather, all inputs that still have cached result for that input.

[medikoo]

@rektide By default, intention is thatmemoizee will only store hash-like information (so not the input arguments in exact form, but unique id that corresponds to them). Having that, what you propose will not be possible directly, but I agree it's a good idea, to have it as an option.