feat: generate `f.eq_unfold` lemmas (#5141)

With this, lean produces the following zoo of rewrite rules:
```
Option.map.eq_1      : Option.map f none = none
Option.map.eq_2      : Option.map f (some x) = some (f x)
Option.map.eq_def    : Option.map f p = match o with | none => none | (some x) => some (f x)
Option.map.eq_unfold : Option.map = fun f p => match o with | none => none | (some x) => some (f x)
```

The `f.eq_unfold` variant is especially useful to rewrite with `rw`
under
binders.

This implements and fixes #5110