Implementation of OpamSysPoll in Dune-terms

[Leonidas-from-XIV]

This branch adds an implementation of the functionality of OpamSysPoll but using our own concurrency primitives.

[rgrinberg]

Do you want to try using this in the rules? Or in a separate PR?

[Leonidas-from-XIV]

I'll try adding this into the rules!

[rgrinberg]

Added @gridbugs to review the solver related stuff

[gridbugs]

Is it necessary to prevent solving packages with repos that don't have system environment variables?

[Leonidas-from-XIV]

Repo_selection.t is a pure value created by parsing command line arguments, so unfortunately at this point I can't inject the environment variables, since that would require evaluating a fiber. Therefore the injection has to be done in a later step. Hence why I added with_sys_env that (in an admittedly rather ugly way) patches in the env later (where possible). But that means that Repo_selection.t with variables and without would be indistinguishable. To alleviate this I introduced a phantom type to force the user to call with_sys_env to turn an empty Repo_selection.t into a with_sys Repo_selection.t.

But a better solution would be more than welcome, because I am rather unhappy with the code as it is now.

[gridbugs]

One thing I would like to preserve is the --opam-env=pure behaviour where the solver is run on a completely empty environment for the purpose of reproducable tests. Also I was thinking of at some point letting the user specify variables to set and I think it would be useful to give the option to set variables starting from a totally empty environment.

[Leonidas-from-XIV]

I've rebased again and squashed the commits. Anything missing still or is this ready to go?