feature gate serde usage

[nicbus]

This PR feature gates serde functionality, fixing clippy and build errors.

While linting the project I ran into these errors reported by clippy --no-default-features.
Interestingly, the errors are not reported if clippy is run with the --workspace option, which is what is used in GH Actions Lints workflow.

I have confirmed the errors are correct trying to build rgb-lib depending on rgb-runtime with the serde feature disabled. This way I get the exact same errors clippy reports without --workspace. I have also checked that it compiles and runs all tests fine when including this PR.

I think the issue with specifying --workspace is that dependency resolution is different. Specifically, rgb-wallet's dependency on rgb-runtime with all enabled features, including serde, might be masking the errors by inadvertently enabling the serde feature on rgb-runtime even if --no-default-features is used.

I am quite confident this might be the case as clippy correctly reports the errors if either cli is removed from the workspace members or the serde feature is removed from rgb-wallet's dependency on rgb-runtime.

I guess the usage of the --workspace option might also be the reason why some GH Actions jobs, like Build > no-default, pass instead of failing (cargo check --no-default-features fails without --workspace on master and passes with this PR).

I think cargo commands need to be run without --workspace, possibly separately for each package, in order to avoid hiding errors.

[dr-orlovsky]

cfg_eval is a PITA: I tried to use it here before - and it was a mess everywhere, so I got rid of it. I will check whether there any other way of fixing the lint and the way dependencies are computed...

[dr-orlovsky]

Maybe I am missing something, but I was able to fix the issue with no cfg_eval, which was unexpected for me. Seems everything works: f9813b7#diff-f643199b78cd917deee88c751028d85bc710961b6f7b874f3c99cf6935fd68b0L109-L118

[dr-orlovsky]

Closing this in favor of fix in ac43eb2