Add more Maybe functions

I'm not expecting immediate merger - I want to get this code out there and let the community discuss potential improvements. `justIf` is a higher-order version of `toMaybe` let-bound in #409.

I don't think anything in here is too controversial, as I left out (1) rewriting `maybe` in terms of `extract` and `map` (2) an unsafe extract function that crashes if given `Nothing`.