lib: add feature memoize

[michaellilltokiwa]

Inspired by idiom 315: https://www.programming-idioms.org/idiom/315/memoization

Would be cool to be able to have this as a subfeature of Unary. So we could do fn.memoize to get a memoized version of a Unary.
But the T of memoize is constrained while Unary's T is not.

[fridis]

Very nice, we need this!
Soon, we might want to use tuples to memoize functions with more arguments. So far, I do not have a good idea for how do do this...
Just maybe change the comment asking for equatable constraint.