`SymbolId` and `ReferenceId` are stored as `NonZeroU32`, but with a wrapper to make `u32::MAX` the illegal value, instead of `0`.
Use the existing `nonmax` crate for this. Our current implementation uses `idx + 1` to avoid the zero value, whereas `nonmax` crate uses XOR `idx ^ u32::MAX`, which is a cheaper operation.
Initially I made this change manually instead of pulling in a dependency, but it's a pain because it requires implementing `Debug` and `PartialOrd` by hand to handle the difference between the "actual" value and its stored representation. So I thought better to use a crate which does this for us.
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