New versioning scheme.

[ejgallego]

After some thinking, I have decided to implement the coq-lsp versioning model for Coq SerAPI. This has a lot of advantages, in particular w.r.t. change tracking.

That means that we will have a single SerAPI version, supporting several Coq versions.

Problem is that we need to move from a scheme where version numbers are 8.19.0+0.19.1 to a scheme where the SerAPI version is first. We could bump the epoch, but as SerAPI is being put in maintenance mode, I think for 8.20 I will provide a coq-serlib package that can be versioned like that, and maybe keep coq-serapi version working as now.

[ejgallego]

Finally this won't be implemented, we have moved to this model in coq-lsp with #409