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

[johnthecat]

Hard to read, for some reason we're not using normal vocabulary like "Enum", "Section/Pallet", etc.
Let's define wording first and then define what we want to do with it. Low level bytes position is part of SCALE spec itself, we need to reuse this abstraction and work with higher level primitives