swift234

Runtime: the Swift heap/zone

Good:
* We now no longer wrap the system malloc zone.
* Tiny allocation quantum is pointer sized, not 16 bytes.
* Far less metadata overhead than the system malloc zone.
Bad:
* Our heap density isn't quite as good as the default zone. This is a O(1)
  versus O(N) tradeoff. The default malloc zone has been trending in this
  direction too.
TODO:
* Introspection
* Tune secondary fast paths
* Huge allocations

Swift SVN r15269