Memory management fixes and cleanup

Fixed limbo/refcounting issue in the XRETURN exception handling
that implements the logic of the RETX* instructions. Objects
returned from within 'try' blocks would get an extra refcount,
causing them to leak.

Refactored eel_o_stringrep(), and added WEAKREFS and LIMBO flags.

Fixed refcounting bug in the 'eel' executive; it would hold on
to the main() entry point, preventing the module from being
unloaded during cleanup.

Added $.__load_binary_module and renamed builtin.__load_binary()
accordingly.

Closes #24 and avoids #23 in eelbox/test.eel.