perf: precise cache for expr_eq_fn

[leodemoura]

This performance issue was exposed by the benchmarks at https://github.com/leanprover/LNSym/tree/proof_size_expt/Proofs/SHA512/Experiments

[leodemoura]

!bench

[leanprover-community-mathlib4-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase c517688f1dc3a7896049c3828d20cb615f7fd470 --onto 8acdafd5b3957382c02779ada451c14da44e2aca. (2024-08-01 01:31:31)

[leanprover-bot]

Here are the benchmark results for commit 34fe8d3.
There were significant changes against commit c517688:

  Benchmark                  Metric          Change
  ============================================================
+ stdlib                     maxrss           -1.1%  (-85.5 σ)
- stdlib                     type checking     7.2% (1307.9 σ)
- tests/bench/ interpreted   maxrss            1.6%  (132.4 σ)