Phase 1 (gc-dev) - Add gc.h, make sure it compiles.
Phase 2 (gc-dev2) - Change how strings are allocated, to clean up the code and be compatible with a new GC algorithm.
Phase 3 (gc-dev3) - Change from using a Cheney-style copying collector to a naive mark&sweep algorithm.
Phase 4 (gc-dev4) - Integrating new tracing GC algorithm, added new thread data argument to runtime.
Phase 5 (gc-dev5) - Require pthreads library, stand cyclone back up using new GC algorithm.

TODO:
- profiling and performance improvements
  let's trying using hash tables for symbol table. maybe give concurrency kit a try,
  since it could be used for atomic operations. also want data structures that will be
  concurrency-friendly, especially hash tables
- need to cooperate when a mutator is blocked
- add_mutation will need to be brought into thread local data. 
- probably exceptions too. anything else?
- multiple mutators, and threading functions/types.  probably want this on a new branch, when ready