Hello,

I have some difficulties with compiler plugins for rewriting typed tree.

The current implementation of `Typemod.type_implementation` behaves
like this (I think):
* Checks types in the original implementation module
* If corresponding  for the implementation `.cmi` file doesn't exist
it writes generated interface to the disk.
* If a `.cmi` file exists  then performs some checks
* Applies typed tree hooks.

The consequence of this behavior is that the plugin can't add new
values to the typed tree, at least they will not appear in the .cmi
file. But adding new values during rewriting the typed tree is exactly
the thing that  I need these days! So, I thought that following
pipeline will suit better (steps 2 and 3 are swapped).

`Typemod.type_implementation`
* Checks types in the original implementation module.
* Calls plugins to rewrite this typed tree and get a new typed tree
and a signature for it
* Creates a `.cmi` file if there are no any or checks that existing
`.cmi` file agrees with the signature returned by plugins' hooks.

I tried to implement this [1] (the important part of the diff is in
the attachment) but  I get a crash of `ocamlrun` when building the
whole compiler. The patch seems harmless but am I missing something
under the hood? Any delayed check that relies on the fact that
signature of a module will not change?  Am I at least trying to fix in
right place? How should I work on crashes like this?


Happy hacking,
Kakadu

[1] https://github.com/Kakadu/ocaml/tree/pptx-fix