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Switch to ppx_deriving 5.0 and ppxlib. Remove the dependency on ppx_tools. Contributed by @kit-ty-kate.

Building the test requires core_bench, which is too time-consuming
and too fragile (difficult to install).

I have run into issues with an opam switch (4.03.0) where ppx_deriving 4.1.5
was installed. I would get an error message from dune claiming that
visitors.ppx cannot be used ("Error: no ppx driver were found").
Upgrading to ppx_deriving 4.4 fixed the problem.