In ppxlib 0.22.0, the AST got bumped to ocaml 4.12. This commit adapts
to the compiler changes from 4.11 to 4.12: mainly, the introduction of
injectivity to type declarations.
Signed-off-by: Sonja Heinze <sonjaleaheinze@gmail.com>

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).