Support ppxlib 0.37 + 0.38

[jmid]

This updates the preprocessor to be compatible with ppxlib 0.37 + 0.38.
The AST changes around function nodes means that this won't build with an earlier version, so I've made 0.37 the lower bound.

I've also updated the dune-bound to require at least 3.22.0

• as that was the version requirement added in Adjust dune bound #48
• stating a duplicate bound with a dune-generated opam-file requires at least 3.22.0 to state (warnings (duplicate_deps disabled))

I've checked locally that this now builds on 5.5 and 5.4.
The test suite cram tests fail there though due to printing differences, so I've added a with-test upper bound for now.

[jmid]

After adding an ocaml < 5.3 upper bound for with-test it is now failing on 5.2 😬