fix: `grind` pattern validation

This PR fixes the `grind` pattern validator. It covers the case where
an instance is not tagged with the implicit instance binder. This
happens in declarations such as
```lean
ZeroMemClass.zero_mem {S : Type} {M : outParam Type} {inst1 : Zero M} {inst2 : SetLike S M}
  [self : @ZeroMemClass S M inst1 inst2] (s : S) : 0 ∈ s
```