chore: update Pi instance names (#5447)

Override instance names for nonempty / inhabited / subsingleton arrows,
per
[zulip](https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/Pi.20instance.20names.20in.20Lean.204.20core/near/466248246).

src/Init/Core.lean

@@ -1897,7 +1897,8 @@ theorem funext {α : Sort u} {β : α → Sort v} {f g : (x : α) → β x}
   show extfunApp (Quot.mk eqv f) = extfunApp (Quot.mk eqv g)
   exact congrArg extfunApp (Quot.sound h)
 
-instance {α : Sort u} {β : α → Sort v} [∀ a, Subsingleton (β a)] : Subsingleton (∀ a, β a) where
+instance Pi.instSubsingleton {α : Sort u} {β : α → Sort v} [∀ a, Subsingleton (β a)] :
+    Subsingleton (∀ a, β a) where
   allEq f g := funext fun a => Subsingleton.elim (f a) (g a)
 
 /-! # Squash -/

src/Init/Prelude.lean

@@ -757,7 +757,8 @@ noncomputable def Classical.ofNonempty {α : Sort u} [Nonempty α] : α :=
 instance (α : Sort u) {β : Sort v} [Nonempty β] : Nonempty (α → β) :=
   Nonempty.intro fun _ => Classical.ofNonempty
 
-instance (α : Sort u) {β : α → Sort v} [(a : α) → Nonempty (β a)] : Nonempty ((a : α) → β a) :=
+instance Pi.instNonempty (α : Sort u) {β : α → Sort v} [(a : α) → Nonempty (β a)] :
+    Nonempty ((a : α) → β a) :=
   Nonempty.intro fun _ => Classical.ofNonempty
 
 instance : Inhabited (Sort u) where
@@ -766,7 +767,8 @@ instance : Inhabited (Sort u) where
 instance (α : Sort u) {β : Sort v} [Inhabited β] : Inhabited (α → β) where
   default := fun _ => default
 
-instance (α : Sort u) {β : α → Sort v} [(a : α) → Inhabited (β a)] : Inhabited ((a : α) → β a) where
+instance Pi.instInhabited (α : Sort u) {β : α → Sort v} [(a : α) → Inhabited (β a)] :
+    Inhabited ((a : α) → β a) where
   default := fun _ => default
 
 deriving instance Inhabited for Bool