Open Radar

 
Summary:
LazySequence should have a `first` property of type `S.Generator.Element?`.

Steps to Reproduce:
1. Try to find the first element matching a given criterion:

    return lazy(start…end).filter { $0 % 2 == 0 }.first

Expected Results:
This code, or something very similar, works fine.

Actual Results:
There is no first property, method, or function that can be used on a LazySequence. The closest equivalent is to use its generator directly, capturing it in a mutable variable first:

    var gen = lazy(start…end).filter { $0 % 2 == 0 }.generate()
    return gen.next()

Notes:
Another alternative would be to add a first() function that takes a SequenceType, but this would be uglier and also would consume elements from one-time-through sequences, which might be unexpected.

Calling `first` twice on a LazySequence might run the `filter` method’s block twice. That would be fine.