Fix panic when throwing arity mismatch

[yihozhang]

Fixes #463

After this change:

❯ echo  "(+ 1 2 3)" | cargo run --release
    Finished `release` profile [optimized] target(s) in 0.20s
     Running `target/release/egglog`
[INFO ] Welcome to Egglog! (build: 0.3.0_2024-12-05_a060316)
[ERROR] All alternative definitions considered failed
      In 1:4-4 of <unnamed.egg>: 1
     Expect expression 1 to have type String, but get type i64
      In 1:1-9 of <unnamed.egg>: (+ 1 2 3)
    Arity mismatch, expected 2 args: (+ 1 2 3)
      In 1:1-9 of <unnamed.egg>: (+ 1 2 3)
    Arity mismatch, expected 2 args: (+ 1 2 3)
      In 1:1-9 of <unnamed.egg>: (+ 1 2 3)
    Arity mismatch, expected 2 args: (+ 1 2 3)

---

Edit: also closes #310

[codspeed-hq]

CodSpeed Performance Report

Merging #477 will not alter performance

_{Comparing yihozhang-fix-arity-panic (3a00eb2) with yihozhang-fix-arity-panic (cbedde5)}

Summary

✅ 8 untouched benchmarks

[oflatt]

Thanks!