Initialize vcs-base refactor

[mbarbin]

Initialize a multi-stages refactoring that will be ongoing in the repo for a while. More info in TODO.md.

The aim of this refactoring is to remove the base dependency of the Vcs library. To achieve this, we will offer two distinct libraries:

• Vcs - a kernel library that can be used with very little dependencies;
• Vcs_base - an extension of Vcs which will add some functionality related to working with Base.

Stages implemented in this PR

Stage 1 - Introducing Vcs_base

In this stage, we create the library Vcs_base and setup the way in which this library extends Vcs. It exposes the same modules, plus extra functionality, such as:

• Base style hash signatures
• Comparable.S signatures for use with Base style containers
• Make some functions return sets instead of lists.

Stage 2 - Reducing ppx dependencies in Vcs

Only keep sexp related ppx that have no runtime dependency on base, such as sexplib0 only.

• Remove ppx_compare, ppx_here, ppx_let dependencies.