mlton: refactor and adds mlton20180207Binary mlton20180207 mltonHEAD

This commit adds mltonHEAD.

Summary of all the changes in this branch:

* mlton20130715
  - for this one I just copy and pasted the old default.nix
  - I have tested some projects that compile with this version and don't
    compile with the newer version, so I think it makes sense to keep
    the old version

* mlton20180207Binary
  - This is used to build the source derivations

* mlton20180207
  - latest release

* mltonHEAD
  - latest commit to the master branch of the github repo at the time of
    creating this derivation

pkgs/development/compilers/mlton/default.nix

@@ -15,4 +15,12 @@ rec {
     sha256 = "00rdd2di5x1dzac64il9z05m3fdzicjd3226wwjyynv631jj3q2a";
     inherit stdenv fetchgit gmp;
   };
+
+  mltonHEAD = import ./from-git-source.nix {
+    mltonBootstrap = mlton20180207Binary;
+    version = "HEAD";
+    rev = "e149c9917cfbfe6aba5c986a958ed76d5cc6cfde";
+    sha256 = "0a0j1i0f0fxw2my1309srq5j3vz0kawrrln01gxms2m5hy5dl50d";
+    inherit stdenv fetchgit gmp;
+  };
 }

pkgs/top-level/all-packages.nix

@@ -7165,9 +7165,10 @@ with pkgs;
   inherit (callPackage ../development/compilers/mlton {})
     mlton20130715
     mlton20180207Binary
-    mlton20180207;
+    mlton20180207
+    mltonHEAD;
 
-  mlton = mlton20130715;
+  mlton = mlton20180207;
 
   mono  = mono5;
   mono5 = mono58;