retire the classic module

With Prosa 0.5 out the door, I think now is a good time to retire the classic module, which is increasingly becoming a drag on maintenance and CI runtimes.

We haven't ported everything of interest yet (see #41, especially this comment), but are also unlikely to do so anytime soon. Hence I think it's best to create as separate prosa-classic branch that contains only the classic module, which then will no longer track Coq and mathcomp version updates. Conversely, the master branch will no longer carry the classic codebase, which will make it easier to stay up-to-date with Coq and mathcomp.

Overall, the classic codebase will remain easily accessible and browsable (over at https://branches.mpi-sws.org) without dragging down the main development.

CC: @proux