Add compactMapValues to Dictionary

[stephencelis]

@lorentey

I think we should have more practical examples to motivate inclusion. Dictionaries of optional values do not occur very often in my experience – I’d typically just declare a non-optional value and filter out nils during insertion. For other cases, a filter followed by mapValues would often work just as well as compactMapValues.

We added a compact method to Dictionary when I was at Kickstarter and used it throughout:

• https://github.com/kickstarter/ios-oss/blob/996ba3d8990c85ac9b47dc15e6362a23af49ed69/KsApi/lib/Route.swift#L119-L123
• https://github.com/kickstarter/ios-oss/blob/996ba3d8990c85ac9b47dc15e6362a23af49ed69/KsApi/lib/Route.swift#L308-L312
• https://github.com/kickstarter/ios-oss/blob/a7636fa76c5c97bc87882ca17212be8e944c795e/Kickstarter-iOS/ViewModels/RootViewModel.swift#L108-L116
• https://github.com/kickstarter/ios-oss/blob/1b21ce9100edc2700b30f41432f4c6077febba69/Kickstarter-iOS/ViewModels/AppDelegateViewModel.swift#L942

I even recently added a filterMapValues (and filteredValues) to our code base at Point-Free:

• https://github.com/pointfreeco/pointfreeco/blob/f97a9e4830a5bdfe778daca8ed771f7fa9eadbc7/Sources/PointFree/Stripe.swift#L306-L311

We try to avoid mutation and write code that's declarative, so our uses tend to be where we want to pass a dictionary literal to a call site that expects non-optional values, but where some of our inputs may be optional.

These use cases may be minor enough to not require an implementation in the standard library, but we'll probably continue to add our own extension and benefit from it regardless. It's a bit messy to filter and mapValues everywhere (especially with the force unwrap). What we want is succinctness and safety.