This PR fixes the `#discr_tree_simp_key` command, because it displays
the keys for just `lhs` in `lhs ≠ rhs`, but it should be `lhs = rhs`,
since that is what simp indexes.
This PR fixes the caching infrastructure for `whnf` and `isDefEq`,
ensuring the cache accounts for all relevant configuration flags. It
also cleans up the `WHNF.lean` module and improves the configuration of
`whnf`.
Adds a command and tactic to print the `Array <| DiscrTree.Key` for
equalities helping the user to debug perceived `simp` failures.
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Co-authored-by: Joachim Breitner <mail@joachim-breitner.de>
