#76: compactMapValues for Dictionaries

When you are transforming the values of a dictionary and some results should be discarded, compactMapValues is perfect.

let raw = ["a": "1", "b": "x", "c": "3"]
let parsed = raw.compactMapValues(Int.init)
// ["a": 1, "c": 3]

It keeps the original keys, transforms each value and drops the entries where the transform returns nil.

That is much cleaner than manually filtering and rebuilding the dictionary yourself.