Split positions of all user-defined predicates into input and outputs

[skeuchel]

The heap predicates have a field that indicates which predicates are precise, and for those it splits up the arguments into inputs and outpus.

katamaran/theories/Syntax/Predicates.v

Lines 56 to 69 in b7d951c

	Module Type HeapPredicateKit (Import B : Base).
	(** Heap Predicates *)
	(* Predicate names. *)
	Parameter Inline 𝑯 : Set.
	(* Predicate field types. *)
	Parameter Inline 𝑯_Ty : 𝑯 -> Ctx Ty.
	(* Duplicable? *)
	#[export] Declare Instance 𝑯_is_dup : IsDuplicable 𝑯.
	#[export] Declare Instance 𝑯_eq_dec : EqDec 𝑯.
	Parameter 𝑯_precise : forall p : 𝑯, option (Precise 𝑯_Ty p).
	End HeapPredicateKit.

I wonder why we went with that definition instead of defining inputs and outputs positions for all predicates uniformly. I think this would improve the code.