- Store a crossgeam GC collector in the BagPipe
- Use this GC instance for pinning, eliminating the dependency on
  the global singleton, and thus on thread-local storage
- elfmalloc: Don't set bsalloc as the global allocator because the
  new GC allocates on clone, and if the global allocator is bsalloc,
  each clone call becomes very expensive, which makes tests run
  very slowly
- elfmalloc: Use opt-level=3 for tests in Travis
- elfc: Set bsalloc as the global allocator

- Now that bagpipes don't rely on TLS, we can safely clean up
  thread-local caches while they are being dropped rather than
  sending pointers to a dedicated background thread