Add defmt, bump heapless, add additional CI build variants, fix serde+no_std

[mchodzikiewicz]

Hi,

I just started using mqttrs for my project, take a look at few changes that I needed to apply.

If you want, I can split it into multiple PRs but the scope is not that big (even though it is not "atomic").

[00imvj00]

@mchodzikiewicz getting this error while running tests, please check.

Compiling mqttrs v0.4.1 (/home/runner/work/mqttrs/mqttrs)
     Running `/home/runner/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/bin/rustc --crate-name mqttrs --edition=2018 src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --emit=dep-info,link -C embed-bitcode=no -C debuginfo=2 --test --check-cfg 'cfg(docsrs)' --check-cfg 'cfg(feature, values("default", "defmt", "derive", "serde", "std"))' -C metadata=72e5bfe7f0d7eadd -C extra-filename=-72e5bfe7f0d7eadd --out-dir /home/runner/work/mqttrs/mqttrs/target/debug/deps -C incremental=/home/runner/work/mqttrs/mqttrs/target/debug/incremental -L dependency=/home/runner/work/mqttrs/mqttrs/target/debug/deps --extern heapless=/home/runner/work/mqttrs/mqttrs/target/debug/deps/libheapless-72d9beaf7b37443a.rlib --extern proptest=/home/runner/work/mqttrs/mqttrs/target/debug/deps/libproptest-82824e0ff27f59cf.rlib`
error[E0432]: unresolved import `bytes`
 --> src/decoder_test.rs:2:5
  |
2 | use bytes::BytesMut;
  |     ^^^^^ use of undeclared crate or module `bytes`
For more information about this error, try `rustc --explain E0[43](https://github.com/00imvj00/mqttrs/actions/runs/12466435357/job/34794956689?pr=49#step:8:44)2`.
error: could not compile `mqttrs` (lib test) due to 1 previous error
Caused by:
  process didn't exit successfully: `/home/runner/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/bin/rustc --crate-name mqttrs --edition=2018 src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --emit=dep-info,link -C embed-bitcode=no -C debuginfo=2 --test --check-cfg 'cfg(docsrs)' --check-cfg 'cfg(feature, values("default", "defmt", "derive", "serde", "std"))' -C metadata=72e5bfe7f0d7eadd -C extra-filename=-72e5bfe7f0d7eadd --out-dir /home/runner/work/mqttrs/mqttrs/target/debug/deps -C incremental=/home/runner/work/mqttrs/mqttrs/target/debug/incremental -L dependency=/home/runner/work/mqttrs/mqttrs/target/debug/deps --extern heapless=/home/runner/work/mqttrs/mqttrs/target/debug/deps/libheapless-72d9beaf7b37[44](https://github.com/00imvj00/mqttrs/actions/runs/12466435357/job/34794956689?pr=49#step:8:45)3a.rlib --extern proptest=/home/runner/work/mqttrs/mqttrs/target/debug/deps/libproptest-82824e0ff27f59cf.rlib` (exit status: 1)

[mchodzikiewicz]

I am sorry, I pushed half-baked commit to this branch instead of a separate one by mistake, pipeline is green again on my fork