Allow borrowed interface types

[jruderman]

Seems weird that interface types are always reference-counted. For example, there could also be a "borrowed" variant, which contains a pointer to a borrowed object and a pointer to a vtable.

(It's possible I'm already confused about the distinction between "how the interface type is allocated" and "how the concrete type is allocated".)

[nikomatsakis]

dup of #1567. In any case, the plan is to have &iface, @iface, and ~iface eventually.