The BIG SWITCH from ocamlbuild to dune.
* Remove support for installing via ocamlfind. * Remove PREFIX. * Move the source of MenhirLib into lib/. * Move the source of MenhirSdk into sdk/. * Build and install everything using dune. * Run all tests using dune. Reorganize the test directory. * Move the old ocamlbuild demos to a subdirectory. * Introduce [make versions] to test Menhir with multiple versions of OCaml. * Update the release scripts.