Preds are just Core preds with ✨flavour✨

[giltho]

Based on #380, look only at the last commit if you're reviewing this before we merge the others.

The core of this PR is this diff:

	type atom = TypeDef__.assertion_atom =
	  | Emp  (** Empty heap *)
-	  | Pred of string * Expr.t list * Expr.t list  (** Predicates *)
      | Pure of Expr.t  (** Pure formula *)
	  | Types of (Expr.t * Type.t) list  (** Typing assertion *)
	  | CorePred of string * Expr.t list * Expr.t list  (** Core assertion *)
	  | Wand of { lhs : string * Expr.t list; rhs : string * Expr.t list }
	      (** Magic wand of the form [P(...) -* Q(...)] *)

Instead, predicates are now represented as core predicates that start with a magic string GILLIAN_USER_PRED__pred_name.
This does not change the instantiations of Gillian (except for compiling user-defined predicates which do that compilation correctly). Similarly, the GIL parser still parses and produces the old version of the code.