Make <- also a unary operator so we can install methods

Now we can use (<- X) := f as syntactic sugar for installMethod

Author: d-torrance

M2/Macaulay2/d/binding.d

@@ -223,7 +223,7 @@ bumpPrecedence();
 bumpPrecedence();
      export ColonEqualW := binaryright(":="); export ColonEqualS := makeKeyword(ColonEqualW);
      export EqualW := binaryright("="); export EqualS := makeKeyword(EqualW);
-     export LeftArrowW := binaryright("<-"); export LeftArrowS := makeKeyword(LeftArrowW);
+     export LeftArrowW := unarybinaryright("<-"); export LeftArrowS := makeKeyword(LeftArrowW);
      export RightArrowW := binaryright("->",arrowop); export RightArrowS := makeKeyword(RightArrowW);
      export DoubleArrowS := makeKeyword(binaryright("=>"));
      export GreaterGreaterS := makeKeyword(binaryright(">>"));