Prefer to work with upstream Lem

[dsheets]

@tomjridge :

Currently we use Thomas' version of Lem. Ideally we would work with the upstream version of Lem.

What are the differences between Thomas' version and upstream Lem? What do we need to do to start working with the upstream Lem? Can we work directly with upstream Lem, or do we need to maintain some diffs?

Are we currently tracking bugfixes to Lem?

@dsheets :

My tests have been against https://bitbucket.org/dsheets/lem which is a fork of Thomas' fork. I've contributed a few of the build system fixes back to the main project. I would very much like to stop having this fork and use the upstream (and get the upstream to incorporate whatever we actually require from them and then produce an actual, numbered, opam-distributed release).