mlton: adds binary release package for mlton version 20180207

This works the same as the binary package in the old derivation, by
patching the release binary. The difference is that this derivation
is also exposed at the top level.

pkgs/development/compilers/mlton/default.nix

@@ -3,4 +3,8 @@ rec {
   mlton20130715 = import ./20130715.nix {
     inherit stdenv fetchurl patchelf gmp;
   };
+
+  mlton20180207Binary = import ./20180207-binary.nix {
+    inherit stdenv fetchurl patchelf gmp;
+  };
 }