Hide lem_zarith modules from the world

[giltho]

lem currently exposes a bunch of modules that have standard names at the top-level. Specifically:

• Bit
• Either
• Vector
• Pmap
• Pset
• ...

This makes lem incompatible with other libraries from the ecosystem that expose modules with the similar name. For instance, one cannot import both a library that depends on Vector, such as Progress, and Cerberus at the same time, see this PR.

Unfortunately, lem isn't built with dune and I'm not quite sure how to navigate the various makefiles to make this change. I also don't want to immediately break all projects based on lem without an easy path to fix. Supposedly, hiding these modules behind a Lem_zarith_std should be enough, and then all projects that actively use these modules can be compiled with the ocaml flag -open Lem_zarith_std