applyCap and applyThreshold

[Martin (Martin R)]

@cukr

I feel like you will usually be capping the value both from the bottom and top, so it should be a one method taking a range. That way you can have a nice, unambiguous name "clamp"

Adding a clamped(to:) method to the Comparable type has been suggested before, e.g. here: [draft] Add `clamped(to:)` to the stdlib .