Move bisect_ppx to instrumentation clause

[psteckler]

A couple of recent PRs have added --conditional to the preprocessing clause of dune files. I had this branch waiting around, so maybe it's time to put the change in, to avoid the need for similar PRs in the future.

In this PR:

• bisect_ppx updated to 2.5.0 in opam.export
• in dune files, move the uses of bisect_ppx from the preprocessing clause to instrumentation
• dune-project files require dune 2.7, which supports this new syntax
• scripts/create_coverage_files.sh passes a flag to dune runtest to create the coverage files when desired, otherwise the coverage files are not generated
• scripts/require-ppxs.py checks that bisect_ppx appears in instrumentation clauses

While this is a big PR, most of the changes are boilerplate. There's also Emacs dune-mode re-indentation of the dune files, which was probably a mistake.

[mrmr1993]

Very nice!