Avoid recursive interfaces

[fasterthanlime]

To be able to deserialize deeply nested records, and also to have "filter" functionality: digging deep into a document, only deserializing what you need from it, not the whole thing, something serde doesn't really let you do.

[fasterthanlime]

So I haven't fully realized the idea yet, but as of #75 the actual deserialize methods are asynchronous, which means we could return Poll::Pending in the middle of everything to avoid overflowing the stack, stash that future somewhere, continue with another future (for deeper deserialization), and resume the other one when we're done.

I think. I need to think more about, uh, how the whole thing would be implemented, because the future that yields would need to give information to the waker on: what type to deserialize, where to deserialize it.

Worth experimenting with further later.

[fasterthanlime]

This has been handled in #96 — recursive interfaces are fine, I did the async thing, it works unreasonably well. I guess I should introduce InfiniteStackDrop and others — because there's the same problem when it comes to freeing them.