Nested interfaces

[macovedj]

Here is an outline for the various ways in which one could describe nested interfaces in wit.

For additional context re: motivation, see this issue, though generally what this enables is the ability to target nested instances which are expressible in wat and binary formats, and not in wit.

An initial implementation that still needs some work can be found here, though at the moment, it would only add support for the case of nesting interfaces from foreign packages.

Perhaps this PR can also be a place to hash out a bit of what the implications would be from a guest lang bindgen perspective as well.