reject #[zeroize(drop)] on struct/enum fields, enum variants

[jessa0]

Reject the #[zeroize(drop)] attribute on struct and enum fields, as users may be missing a zeroizing Drop implementation
they expected to be generated for them.

For consistency, the attribute is now rejected on enum variants as well, which was previously accepted and generated a
Drop impl for the entire enum, but which may not be what users may have expected, as it could be interpreted that such
an attribute would zeroize only the variants containing the attribute on Drop.

Closes #837.

[jessa0]

hmm right, ok. I'm processing the #[zeroize(drop)] attribute twice for structs. i guess the existing unit tests didn't test the parsing at all, so i guess i'll also add a success test case here.

[codecov-commenter]

Codecov Report

Merging #847 (52f3c65) into main (6d9b224) will increase coverage by 1.14%.
The diff coverage is 85.41%.

@@            Coverage Diff             @@
##             main     #847      +/-   ##
==========================================
+ Coverage   52.75%   53.90%   +1.14%     
==========================================
  Files          82       82              
  Lines        2942     2985      +43     
==========================================
+ Hits         1552     1609      +57     
+ Misses       1390     1376      -14     

Impacted Files	Coverage Δ
zeroize/derive/src/lib.rs	83.52% <85.41%> (+50.19%)	⬆️

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[jessa0]

ugh ok, ran cargo fmt on the latest commit and force-pushed

[tony-iqlusion]

Looks good. Thank you!