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