Remove unused 'serde-1' attribute

[DaniPopes]

The feature "serde-1" doesn't exist.

The other change is a warning issued on a more recent rust version:

warning: elided lifetime has a name
  --> src/function/index.rs:65:40
   |
62 |     pub(crate) fn to_canonicalize<'a>(
   |                                   -- lifetime `'a` declared here
...
65 |     ) -> impl Iterator<Item = usize> + '_ {
   |                                        ^^ this elided lifetime gets resolved as `'a`
   |
   = note: `#[warn(elided_named_lifetimes)]` on by default

[codspeed-hq]

CodSpeed Performance Report

Merging #465 will not alter performance

_{Comparing DaniPopes:unused-serde1 (d723300) with main (2c8d947)}

Summary

✅ 8 untouched benchmarks

[saulshanabrook]

I had noticed this as well. I guess maybe originally there was some hope to make the union find serializable. I think at this point that has been superseded by the e-graph serialization format.

Thank you!