20130715.nix is exactly the same as the old default.nix other than
pulling the meta data out into its own file.

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.

Updates from-git-source.nix to use the one-liner to fix up the bash
shebang lines.

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

mlton: refactor and adds mlton20180207Binary mlton20180207 mltonHEAD