As it turns out, the correct way to handle this is much simpler than what I
did originally.
Also add more tests.

In trans_bind_thunk, we need to load the tydescs from the environment *before*
we try to construct derived tydescs from them.

This still doesn't work with bare fn arguments with type parameters.

snapshot.

Closes #469.

This makes getting the function fields out of the visitor less
cumbersome and more efficient (no take/drop).

Closes #619

You can now say

    expr_move(?dst, ?src) | expr_assign(?dst, ?src) { ... }

to match both expr_move and expr_assign. The names, types, and number
of bound names have to match in all the patterns.

Closes #449.

The alias rules guarantee that the alt-ed value will outlive the binding's
last use, so this is safe.

This is a preparation for or-patterns. Joining scoped bindings seems like
it would get messy.

At long last, this patch makes #fmt usable from inside the standard library.
The way it does it us very hackish, but at least it works now.

rt: Remove the now-unused upcall_ivec_resize and upcall_ivec_spill, which allocated from the wrong heap

Somehow this has been disabled forever