`filter` is doing too much work

[robrix]

Swift.filter is strict, meaning that it constructs an intermediate collection. We only need forward iteration via SequenceType, so lazy() should be a significant speedup since it won’t allocate as much.

I hypothesize that this will be significant with short sets and accept-all predicates, and even more significant with long sets. I further hypothesize that it will be insignificant with reject-all predicates.