make Format trait dyn compatible

[jordens]

• Format::_format_tag() -> Format::_format_tag(&self)
• change the wire format for empty slices a usize(0)
• change the wire format for empty arrays to be empty

closes: #861

[jonathanpallant]

Someone can now write:

let slice: &[&dyn defmt::Format] = &[&1.0, &2];
defmt::info!("{}", slice);

But I see:

INFO [1.0, 1e-323]

I think this is because the bits for 2i32 have been interpreted as an f32.

[jordens]

Yeah. That's problematic.

[jordens]

After a lengthy discussion in the chat:

There is a way to correctly support [&dyn Format] (and [Box<dyn Format>] etc) along the lines of
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=08d81d35efe5c2cbe41b614f409bc72c

But it requires restricting the blanket impl Format for & (and &mut, Box etc) to only sized types. Instead one would need to explicitly add impls for such containers of known unsized types: dyn Format, [T], and str (maybe others?).

It's unclear to me whether that's acceptable. If not, then this PR can be closed.

[jonathanpallant]

That would count as a breaking change for anyone currently relying on that impl, and so I don't think we can do that. Thank you for digging through the details though and putting together this PR. It is really appreciated.

[jordens]

That leaves the change to the wire format omitting the tag for empty slices and arrays. What are the prospects there?

[jordens]

#929