__fini__

[Ori Bernstein <ori@xxxxxxxxxxxxxx>]

Myrddin has supported '__init__' functions called after main for a long time.

I just committed '__fini__' functions, which work much the same way but are
called after main. There are a couple of bugs left to fix -- specifically,
they should be called in reverse dependency order, and they should be
triggered by `std.exit()`.

Documentation and bugfixes are coming.

I added them so we could do things like waiting for all threads to exit
in libthread, but I'm sure there are other places they'll be handy.

-- 
Ori Bernstein <ori@xxxxxxxxxxxxxx>