Port the Memoize view into xilem_core

[Philipp-M]

This ports the memoize view into xilem_core as macro.

I haven't added documentation yet (this could probably be its own PR I think...), it's basically just a copy of the original prototype, adjusted to be compilable again.
I've added two functions, one for a static view and one that just wraps the Memoize view (see it in action e.g. here or here)

I guess these extra functions (including their names) are likely a point to discuss still. Especially the static view function could probably be a macro (but I think the syntactic sugar benefits are marginal, by avoiding the extra closure)

[derekdreery]

LGTM

We should standardize naming/location in a follow-up PR. I think the generate_ prefix is the better name, but putting the macro in a view submodule is tidier.