Add support for UIFontMetrics

[noremac]

UIFontMetrics was announced as a new feature in iOS 11 to help deal with custom fonts and dynamic type. It looks like it accomplishes something similar to the various adapt functions in BonMot.

Here's an example from one of the session videos (no VOD link yet)

let bodyMetrics = UIFontMetrics(forTextStyle: .body)

let standardFont = ... // any font you want at the standard type size
let font = bodyMetrics.scaledFont(for: standardFont)

More can be read about it here.