Mapping continuations.

Adds top-level map and flatMap to map between continuations expecting different result types.

Also adds appending to Arrays for when we’d rather chain a function rather than use the + operator.

Adds Index, Count, and weak BindIfReady.

Also pulls out memoized and lazy wrappers like Forgetful for now. They cause Xcode 11 to crash.

Moves from Carthage to Swift Package Manager

v2.0.0

Moves exclusively to SPM.