Replace custom "holes" AST nodes by extensions to restore binary compatibility

[voodoos]

There is currently an issue with Merlin's new typed holes and ppxes. Since these holes were added as new AST node ppxes crash in there presence.

This PR removes these new parsetree nodes and restore the old extension-based ones. We still keep the typedtree nodes that improve the typing if these holes.

I added a test with a custom ppx that shows the error. However this requires to have ppxlib as a test-only dependency for the merlin package. @trefis I know you were wary about doing that, but it really looks like the easiest way to test ppxes in the the testsuite, do you see another option ?

cc @lpw25