GitLab

Move everything into the new namespace starting with 'prosa' rather
than the bland 'rt'.

The main restructuring thrust is nearing completion, so let's get rid
of the `restructuring` namespace.

For improved parallelism and nicer documentation.

Now that we have non-ambiguous dependencies, parallel builds should
work without issue.

If there are breaking changes coming in ssreflect, it would be nice to
know early.

We no longer support Coq 8.8.

Add the `htmlpretty` target to the Makefile to generate prettier documentation,
based on the CoqdocJS project.

     https://www.ps.uni-saarland.de/~ttebbi/coqdocjs/

     https://github.com/tebbi/coqdocjs

Many thanks to Tobias Tebbi for creating CoqdocJS.

Coqdoc produces really nice output - let's automate this.

There's no need to run this for every compiler version; we only care
about the "main" version.

This avoids having to compile ssreflect from scratch each time we want
to compile Prosa.

Thanks to Pierre Roux (Pierre.Roux@onera.fr) for pointing out the
mathcomp Docker images!

From the mathcomp 1.9.0 release notes:

> removed Coq prelude hints plus_n_O plus_n_Sm mult_n_O mult_n_Sm, to
> improve robustness of by ...; scripts may need to invoke addn0,
> addnS, muln0 or mulnS explicitly where these hints were used
> accidentally.

=> This patch makes these required fixes in Prosa.

While at it, turn on CI for coq:dev and Coq 8.9 with two versions of ssreflect.

Using the official Coq Docker images kindly provided by the Coq community:

	https://github.com/coq-community/docker-coq/wiki/CI-setup

Using the following CI template as a starting point:

	https://gitlab.com/erikmd/docker-coq-gitlab-ci-demo-1/blob/master/.gitlab-ci.yml