flambda-backend: Expose a couple more functions from Pprintast (#1731)

This way they can be used by Ppxlib

Author: antalsz

parsing/pprintast.ml

@@ -2003,7 +2003,9 @@ let class_field = class_field reset_ctxt
 let class_type_field = class_type_field reset_ctxt
 let class_expr = class_expr reset_ctxt
 let class_type = class_type reset_ctxt
+let class_signature = class_signature reset_ctxt
 let structure_item = structure_item reset_ctxt
 let signature_item = signature_item reset_ctxt
 let binding = binding reset_ctxt
 let payload = payload reset_ctxt
+let type_declaration = type_declaration reset_ctxt

parsing/pprintast.mli

@@ -49,3 +49,6 @@ val structure_item: Format.formatter -> Parsetree.structure_item -> unit
 val signature_item: Format.formatter -> Parsetree.signature_item -> unit
 val binding: Format.formatter -> Parsetree.value_binding -> unit
 val payload: Format.formatter -> Parsetree.payload -> unit
+
+val class_signature: Format.formatter -> Parsetree.class_signature -> unit
+val type_declaration: Format.formatter -> Parsetree.type_declaration -> unit