Add `rejectIf`

[dggluz]

Description

Handy to do:

findUser(userId)
    .chain(rejectIf(x => !x.length, new UserNotFoundError(userId))) 

Note: as the predicate is sync, it would be dangerous as you can pass a predicate to a Task<boolean, E> that will actually always be cast to true and the type system will not warn you. We should force the predicate - via typings - to be a sync predicate (a function to a boolean value).

We could also make another version for "tasked" predicates.