Add support for key paths?

[zoul]

Key paths would make a nice addition to the API, for example on Signal:

func filterMap<U>(_ path: KeyPath<OutputValue, U?>) -> Signal<U> {
    return filterMap { $0[keyPath: path] }
}

Hopefully we will eventually get some kind of keypath ↔︎ function equivalence on the language level, but until then, would you consider adding the overloads for key paths where appropriate?

[mattgallagher]

Sure. I'll try to do this later this week.

[mattgallagher]

Wow, my ability to predict the future is terrible. In any case... I've added keyPath in commit 7e67484b7d49e7fa7b67e29c217b9b602e13332f

If you also need to filter optionals, you can follow with compact().