Windows binary is missing `.exe` extension

[Timmmm]

If you install this on Windows it puts a binary in C:\Users\Tim\AppData\Local\opam\default\bin called lem, but it should be lem.exe. That means it isn't found if you try to run it from a console, though it does seem to work inside other opam commands (I guess it runs everything in cygwin or whatever).

I think this comes from here. I'm kind of surprised you have to do this step manually. Is this something that Dune would solve?

[Alasdair]

I think the fix is to switch to dune and ditch the existing build system. Would likely help with fixing #39 as well.