Make Ephemeron compatible with infix pointers.

[jhjourdan]

This fixes #9485 and #7810.

Cc @bobot, @damiendoligez.

[jhjourdan]

I am not completely sure I did not miss any patch, perhaps, @bobot, you could check a bit more.

[jhjourdan]

Since this is a bugfix, this could be back ported to 4.11. That said, given that this bug exists for a long time for weak pointers and nobody was really annoyed, this may not be necessary.

[xavierleroy]

@bobot, @damiendoligez : this is a bug fix for an area of the run-time system that you are responsible for. Please review.

[bobot]

Thank you for the heads up. I think one case remains to be done. However in #9424 I will need to add this infix handling in many places.

[xavierleroy]

@bobot are you happy with the revised PR? @damiendoligez would you care to comment?

[bobot]

I approve

[xavierleroy]

Thank you gentlemen, I'll merge now. It's too late for a backport to 4.11, this will wait until 4.12.