Allow renaming of defmt in Format derive

[jamesmunns]

In the postcard ecosystem, I've started gating all deps behind versioned features, e.g. defmt-v0_3 instead of defmt. I typically rename the package to allow multiple incompatible versions to exist.

I noticed that the Format macro does not like this:

#[cfg_attr(feature = "defmt-v0_3", derive(defmt_v0_3::Format))]
pub struct Key([u8; 8]);

leads to a compilation error:

error[E0433]: failed to resolve: use of undeclared crate or module `defmt`
  --> source/postcard-schema/src/key/mod.rs:26:43
   |
26 | #[cfg_attr(feature = "defmt-v0_3", derive(defmt_v0_3::Format))]
   |                                           ^^^^^^^^^^^^^^^^^^ use of undeclared crate or module `defmt`
   |
   = note: this error originates in the derive macro `defmt_v0_3::Format` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0433`.

Serde allows for something like:

#[derive(serde_other::Serialize, serde_other::Deserialize)]
#[serde(crate = "serde_other")]
struct Example { }

This might be nice to have for defmt as well.

[jonathanpallant]

Oof. The expansion of the macro is relying on defmt::_stuff() working.

I guess we need that extra attribute to rename what it expects.

[jonathanpallant]

Although it should be noted that we're going to a lot of trouble to make sure this kind of import naming will never be required for defmt, because 0.3 will have a semver-trick crate to import-export defmt-1.0.

[jamesmunns]

Yep, can't fault you if you decide not to prioritize this for that reason. I was looking to add defmt 0.3 today, and 1.0 later.

Serde's macro expansion has one level of indirection, instead of serde:: it does #serde::: https://github.com/serde-rs/serde/blob/04ff3e8f950eb9862759781d85ec29d906917b70/serde_derive/src/ser.rs#L26-L55