This PR changes the construction of a `CompleteLattice` instance on
predicates (maps intro `Prop`) inside of
`coinductive_fixpoint`/`inductive_fixpoint` machinery.
Consider a following endomap on predicates of the type ` α → Prop`:
```lean4
def DefFunctor (r : α → α → Prop) (infSeq : α → Prop) : α → Prop :=
λ x : α => ∃ y, r x y ∧ infSeq y
```
The following eta-reduced expression failed to elaborate:
```lean4
def def1 (r : α → α → Prop) : α → Prop := DefFunctor r (def1 r)
coinductive_fixpoint monotonicity sorry
```
At the same time, eta-expanded variant would elaborate correctly:
```lean4
def def2 (r : α → α → Prop) : α → Prop := fun x => DefFunctor r (def2 r) x
coinductive_fixpoint monotonicity sorry
```
This PR fixes the above issue, by changing the way how `CompleteLattice`
instance on the space of predicates is constructed, to allow for the
eta-reduced case, as outlined above.
