Add support for OCaml 5.1

[jchavarri]

Besides the changes due to AST updates, the main source of changes originates in ocaml/ocaml#1391. That change leads to errors of the kind:

ocamlfind ocamlopt -c -for-pack Camlp4.Struct -g -safe-string -w Z -I camlp4/import -package camlp-streams -pp 'camlp4/boot/camlp4boot.byte -D OPT' -I camlp4/Camlp4/Struct -I camlp4/Camlp4 -I camlp4/config -I camlp4 -o camlp4/Camlp4/Struct/Loc.cmx camlp4/Camlp4/Struct/Loc.ml
File "camlp4/Camlp4/Struct/Loc.ml", line 1:
Error: camlp4/Camlp4/ErrorHandler.cmx
       was built with -for-pack Camlp4, but the
       current unit Loc is built with -for-pack Camlp4.Struct
Command exited with code 2.

The solution found is to move some modules inside Struct pack (and append a suffix to avoid issues with duplication) and just include them from the outside so the public interface remains unchanged.

There might be some breaking changes though, for example, Camlp4Ast changed its location.

Thanks @kit-ty-kate and @ygrek for the help.

[Octachron]

Note that with ocaml/ocaml#12609 soon to be merged in 5.1, the changes to the structure of camlp4 is not needed anymore.

[kit-ty-kate]

The extprot testsuite now passes. I can do the release if that sounds good for you all?

[ygrek]

yes please!