This PR changes the interface of the `ForIn`, `ForIn'`, and `ForM`
typeclasses to not take a `Monad m` parameter. This is a breaking change
for most downstream `instance`s, which will will now need to assume
`[Monad m]`.
The rationale is that if the provider of an instance requires `m` to be
a Monad, they should assume this up front. This makes it possible for
the instanve to assume `LawfulMonad m` or some other stronger
requirement, and also to provided a concrete instance for a particular
`m` without assuming a non-canonical `Monad` structure on it.
Zulip: [#lean4 > Monad assumptions in fields of other typeclasses @
💬](https://leanprover.zulipchat.com/#narrow/channel/270676-lean4/topic/Monad.20assumptions.20in.20fields.20of.20other.20typeclasses/near/537102158)
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.
Adds a command and tactic to print the `Array <| DiscrTree.Key` for
equalities helping the user to debug perceived `simp` failures.
---------
Co-authored-by: Joachim Breitner <mail@joachim-breitner.de>
