Avoid explicit versions in migrate_parsetree_x_y_migrate.ml

[xclerc]

The current sources of the following files use explicit versions numbers for e.g. constructors in conversion functions:

• src/migrate_parsetree_408_409_migrate.ml;
• src/migrate_parsetree_409_408_migrate.ml;
• src/migrate_parsetree_409_410_migrate.ml;
• src/migrate_parsetree_410_409_migrate.ml.

This pull request replaces references to Ast_4?? with references to From/To (which are defined at the top of the files).

This is sounds useless, but makes the aforementioned files consistent with the previous ones, which in turn simplies reviews.

[hhugo]

You could also try to patch https://github.com/ocaml-ppx/ocaml-migrate-parsetree/blob/master/tools/gencopy.ml. It is the tool responsible for generating theses files.

[ghost]

That's a nice change. Agreed with @hhugo that the tool should be patched as well, so that we use the new convention moving forward.