Generalize malloc-via-gc-heap support, make it available to unix port.
unix: Implement garbage collection support.
stm: Provide malloc()/realloc()/free(), as forwarding to gc_*() functions.

Note: none of these functions are used by STM port, so they're provided
to have parity with pre gc refactor status quo, and in case they will be
needed in the future by some extension writers.
More GC debugging improvements.
Merge pull request #278 from pfalcon/unix-gc

Enable GC for Unix port