reason-parser: add interface files for Reason_parser_{def,explain}

Author: anmonteiro

src/reason-parser/reason_parser_def.mli

@@ -0,0 +1,12 @@
+open Ppxlib
+
+type labelled_parameter =
+  | Term of Asttypes.arg_label * Parsetree.expression option * Parsetree.pattern
+  | Type of string
+
+type let_bindings =
+  { lbs_bindings : Parsetree.value_binding list
+  ; lbs_rec : Asttypes.rec_flag
+  ; lbs_extension : (Parsetree.attributes * string Asttypes.loc) option
+  ; lbs_loc : Location.t
+  }

src/reason-parser/reason_parser_explain.mli

@@ -0,0 +1,6 @@
+(* See the comments in menhir_error_processor.ml *)
+
+val message :
+   'a Reason_parser.MenhirInterpreter.env
+  -> Reason_parser.token * 'b * 'c
+  -> string