Decode macro variable name clashes

[nberrington-cloudian]

I have a struct with a field name called tag that caused a compile error in the musli::Decode macro.

This is because the generated code also uses a variable called tag to decode field tags, shadowing the tag variable used to hold the decoded field contents.

I suggest the mutable field variables should have something like a field_ prefix to disambiguate them from the variables used by the decode function.

Happy to submit a PR if you're in agreement.

[udoprog]

Yeah, I agree. The variable names should probably go through the ident method which exists exactly for this reason.

Feel free to make a PR!