Streamed read for binary HTTP

[martinthomson]

This is far from ready.

It also requires an MSRV update to 1.82, which is brand new, so I don't want to say that this is ready to land.

Missing:

• The writing part. It is likely possible to cobble something together from the synchronous API easily, so I didn't spend too much time on this, but that will have to be worked out.
• Testing. The tests I have are basic smoke tests. I found a lot of missing coverage in a short time.
• ohttp parts. That part should be fairly simple (it can probably reuse a lot of the Body implementation), but this is still likely to be non-trivial.