Updates uthash to 1.9.9
WIP: Fixes GC bugs, adds additional GC profile/debug tools

- First steps toward a real garbage collector. Fixes some big issues
  with mark and sweep. There are still some other GC issues that crop up
  when it's run many times, particularly with large callstacks caused by
  recursion. I'm working on additional improvements.
Merge branch 'master' of github.com:ndreynolds/flathead

Conflicts:
	src/nodes.c
Don't empty hash for now, remove added Makefile option

- Emptying the hash in GC sweep phase leads to a segfault in one of the
  test cases. It's possible that the code is trying to access a hash
  after the corresponding js_val is freed. Needs to be further debugged.