Improve usage and support for immutable `AbstractVarInfo`

[torfjelde]

After #1726 we can make use of alternative implementations of AbstractVarInfo, in particular immutable implementations. This comes with a lot of benefits, with a big one being performance improvements for a large suite of models.

But current a lot of the codebase assumes AbstractVarInfo to be mutable, which first needs to change. This issue is for keeping track of what needs to be done.

[yebai]

Fixed by #1900