Adding firstAs() to Sequence

[timv (Tim Vermeulen)]

@David_Catmull

That's another one that I have under a slightly different name:

func firstResult<T>(_ predicate: (Element) -> T?) -> T?
{
  return lazy.compactMap(predicate).first
}

Fun fact! This version isn't actually lazy. LazySequenceProtocol.compactMap takes an escaping closure, which this predicate isn't, so the compactMap is silently inferred to be the regular Sequence.compactMap.