Re: Rewrite merlin-completion-at-point integration to be faster and better

[voodoos]

The first merge tentative of @catern's rewrite of completion-at-point (CAP) lead us to discover a few issues that need fixing before we release it. I reverted the commits and this PR reintroduces changes made by @tmcgilchrist and myself to move the tests in a separate file, fix linting issues and enable testing on version 27 to 29.

There are two issues with the current status of this PR:

• Even when using the compat module tests do not pass on Emacs 27 and 28.
• Using the compat module breaks the assumption made by opam user-setup that packages are standalone.

This last issue could be fixed by: 1. shipping the compat package with Merlin or 2. Rewriting our own replacement for the missing features.