Separate "crate version" from "wire format version"

[Dirbaio]

Depends on #539. Fixes #287.

This decouples the "wire format version" from the crate version (Cargo package version or git commit). Wire format version can now be bumped independently. This alleviates pains listed in #539., and more importantly it will allow making breaking changes to the wire format in a semver-minor defmt crate release. See #426.

Version is 3 to be able to retroactively number 0.1.x = 1, 0.2.x = 2. A followup PR will add support to defmt-decoder to decode both version 2 and 3 concurrently.

[Urhengulas]

Hi @Dirbaio, Can you please resolve the merge conflict? 😁

[Urhengulas]

Heh, your change makes #518 pretty-much obsolete 🙈 I will revert it (#545).

[jonas-schievink]

Looks good, though some in-code docs would be good to have here, since this is quite important to get right.

[jonas-schievink]

bors r+

[bors]

Build succeeded:

• ci