This commit splits gc.c into two files:

- gc.c now only contains code not specific to Ruby GC. This includes
  code to mark objects (which the GC implementation may choose not to
  use) and wrappers for internal APIs that the implementation may need
  to use (e.g. locking the VM).

- gc_impl.c now contains the implementation of Ruby's GC. This includes
  marking, sweeping, compaction, and statistics. Most importantly,
  gc_impl.c only uses public APIs in Ruby and a limited set of functions
  exposed in gc.c. This allows us to build gc_impl.c independently of
  Ruby and plug Ruby's GC into itself.