Implement foreign objects

[Javyre]

I half-wrote a system for foreign object using finalizers but the manual strongly advises against that...

We could try to use scm_gc_malloc() with boxes or something which would be way cleaner...
heres an example usage of scm_gc_malloc() and scm_gc_malloc_pointerless(): https://www.gnu.org/software/guile/manual/guile.html#Creating-Foreign-Objects

[Javyre]

According to guile guys, finalizers are fine since we need destructors when dropping rust objects...
Ill start working on the seconds half of the impl

[Javyre]

Well, I successfully implemented foreign objects pretty cleanly with only one slot...
still waiting on response from guile-user: https://lists.gnu.org/archive/html/guile-user/2018-02/msg00003.html

[Javyre]

Left a lot of the old Typelist code commented out in the commit since I don't know if this is the definitive solution... Still no response from mailing list... 😢