RFC 0028: Nested Wire Envelope: Trait, Method, Version, and Direction

[decrypto21]

Adds the design proposal for folding wire direction (request/response, or a subscription's start/stop/interrupt/receive) and version into the payload itself, so each method costs one wire id instead of two or four. Registers it in docs/rfcs/_index.md.

Implemented directly in #357 rather than a separate PR: neither has merged, and the nested envelope isn't separable from that PR's own wire redesign, same unreleased codec 2 cutover.

• Doc: docs/rfcs/0028-nested-wire-envelope.md
• Implementation: feat(wire): split the frame discriminant into trait and method bytes #357

[johnthecat]

I think Receive should be after Start

pub enum Subscription<Start, Item, Err> {
    Start(Start),
    Receive(Item),
    Interrupt(Option<Err>),
    Stop,
}

So subscriptions can be partly backward compatible with request