Benchmarks: use Ppxlib to generate code instead of compiler-libs

What

Lib_benchmark depends on the parsetree to generate code. Unfortunately, som constructors have been removed in OCaml5. This MR rewrites the AST generation using Ppxlib.

Why

This makes it compilable with both OCaml 4.14 and 5, as PPxlib deals with the compatibility layer.

How

Manually testing the MR

As usual, from a switch with OCaml5 you can simply try make octez-snoop with and without this patch . The tests are checking that the generated code is correct with %expect and regressions, so there are no changes.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR