juliac: Add basic `finalizer` support
This implementation is fairly sloppy (and also limited), but it's
sufficient to get the finalizer functions for I/O implemented and
working, which is pretty important to make sure that files get closed,
etc.
For a proper implementation later, we probably want to move the method
lookup to type-inference and have it emit a `Core.finalizer_invoke`
intrinsic for us to use in codegen, which would allow us to handle more
cases where e.g. `inlining` would like to union-split the finalizer
invoke or the target function has a single method that covers us.
`atexit()` will likely need a similar trick to "lift" the dispatch that
it defers too.
