Consider treating LazyFilteredCollection like FlattenCollection

The refusal to pre-count elements of a `FlattenCollection` is based on reasoning about the performance of `flatMap`. But `flatMap`, when applied to collections of optionals, doesn't produce a LazyFlattenedCollection<…>, but a LazyMapCollection<LazyFilterCollection<…>>. We are not doing a similar thing for LazyFilterCollection.

The following demonstrates:

struct Foo {
  let i: UInt ;
  init?(_ i: UInt) { print("Foo(\(i))"); if i % 4 == 0 { return nil }; self.i = i }
}
Array((0..<10).lazy.flatMap(Foo.init))

which currently generates Foo(i) 3 times for each i in 0..<10. If this FIXME were addressed it would go down to 2 times (once for counting), but if we take the option suggested in this bug, it would go down to 1 (at the cost of Array reallocations due to failing to pre-count, of course).