Previously, the `UserData` and `LocalUserData` classes were only supposed to be
used by the lazy-function system. However, they are generic enough so that they
can also be used by the multi-function system. Therefore, this patch extracts
them into a separate header that can be used in both evaluation systems.
I'm doing this in preparation for being able to pass the geometry nodes logger
to multi-functions, to be able to report errors from there.
Pull Request: https://projects.blender.org/blender/blender/pulls/138861
6d9b9dd2c3