Fix off-by-one bug in serde impl for Symbol types

[caspark]

The various Symbols' serde implementations had a bug that would cause their underlying index to increase by 1 each time they're serialized and deserialized, because the serialize impl didn't account for the fact that Symbol::new() adds one to guarantee that the resulting index is non-zero.

This fixes that and adds tests for it.

• A less obtuse fix would be to move the + 1 operation out of Symbol::new() and into the deserialize impl for each Symbol, but I'm not sure if something else in this codebase relies on Symbol::new() adding 1.
• Wasn't sure whether you want tests in serde_impl.rs or in the existing integration test binary; I opted for the former because it means the tests will neatly be automatically disabled if the serde feature is off.

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Fixes #88 .

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.34%. Comparing base (e92b6b6) to head (98b2451).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
+ Coverage   60.60%   61.34%   +0.74%     
==========================================
  Files          11       11              
  Lines         500      507       +7     
==========================================
+ Hits          303      311       +8     
+ Misses        197      196       -1     

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[Robbepop]

Thanks for the intended fixes. I have reviewed the changes. Please fix the nits I have. Ideally we could even make the Symbol (de)serialization more generic by implementing (De)Serialize for all T: Symbol.

[caspark]

Feedback should be addressed now. Swapped to using from_usize() in the Deserialize impl too, to clearly mirror the Serialize impl.

We can't impl serde traits for all T: Symbol unfortunately because of the orphan rule: it would cause overlapping trait impls for those traits - e.g. usize would have our impls and serde's impls.

[caspark]

Latest feedback applied!

[Robbepop]

@caspark Thanks a lot for this!

[caspark]

No worries, thanks for the quick reviews!