Relax patterns authorized on let bindings :

let x & t = is now allowed.

lang/parser/parser.mly

@@ -697,9 +697,15 @@ let_binding:
 ;
 
 ident_or_let_pat:
+
+| p1 = ident_or_let_pat "&" p2 = ident_or_let_pat_constr { mk $sloc (And(p1, p2)) }
+| p = ident_or_let_pat_constr { p }
+;
+
+ident_or_let_pat_constr:
 | id = located_ident { mk $sloc (PatVar [ (snd id) ]) }
 | p = constr_pat { p }
-| p1 = ident_or_let_pat "&" p2 =constr_pat { mk $sloc (And(p1, p2)) }
+
 ;
 
 branches_: