unix: Enable GC.

GC support for "unix" port was implemented some time ago, but still not
enabled.
Merge pull request #417 from pfalcon/unix-enable-gc

unix: Enable GC.
py: More robust int conversion and overflow checking.
py: Add "io" module.

So far just includes "open" function, which should be supplied by a port.

TODO: Make the module #ifdef'ed.