This PR implements the proof terms for the new `grind ac` module.
Examples:
```lean
example {α : Sort u} (op : α → α → α) [Std.Associative op] (a b c d : α)
: op a (op b b) = op c d → op c (op d c) = op (op a b) (op b c) := by
grind only
example {α : Sort u} (op : α → α → α) [Std.Associative op] [Std.Commutative op] (a b c d : α)
: op a (op b b) = op d c → op (op b a) (op b c) = op c (op d c) := by
grind only
example {α : Sort u} (op : α → α → α) [Std.Associative op] [Std.Commutative op]
(one : α) [Std.LawfulIdentity op one] (a b c d : α)
: op a (op (op b one) b) = op d c → op (op b a) (op (op b one) c) = op (op c one) (op d c) := by
grind only
```
The `grind ac` module is not complete yet, we still need to implement
critical pair computation and fix the support for idempotent operators.
c4e5f57512