Start thinking about streaming deserialization

[fasterthanlime]

both from_read and from_async_read — I think we can actually do both!

(neither of these would be able to borrow, but that's fine).

[fasterthanlime]

This will likely have to wait for N+2? Changing the JSON/YAML/MessagePack deserializers so they can operate from "not-a-slice" is kind of annoying, most likely.

...but N+1 will have "streaming serialization", so that's nice.