btree_gist into core?

To use exclusion constraints in practice, you often need to install the
btree_gist extension, so that you can combine for example a range type
check and normal scalar key columns into one constraint.

The currently proposed "application time" feature [0] (also known more
generally as temporal database) is in many ways essentially an
SQL-syntax wrapper around typical use cases involving ranges,
multiranges, and exclusion constraints. As such, it also needs
btree_gist installed in most (all?) cases. I have argued over in that
thread that it would be weird to have a built-in SQL feature that relied
on an extension to work at all. So I think the way forward would be to
move btree_gist into core, and I'm starting this new thread here to give
this topic a bit more attention.

So, first of all, would people agree with this course of action?

I don't have a lot of experience with this module, so I don't know if
there are any lingering concerns about whether it is mature enough as a
built-in feature.

If we were to do it, then additional discussions could be had about how
to arrange the code. I suspect we wouldn't just want to copy the files
as is under utils/adt/, since that's a lot of files.

There are also of course questions about how to smoothly arrange
upgrades from extensions to the built-in situations.

Thoughts?

[0]:
https://www.postgresql.org/message-id/flat/CA+renyUApHgSZF9-nd-a0+OPGharLQLO=mDHcY4_qQ0+noCUVg(at)mail(dot)gmail(dot)com