fix: lifted mutables become immutable

[yoav-steinberg]

Fixes broken test and libs due to winglang/wing#6303

• Lifted mutables become immutable, so we can't treat them an mutable.
• Don't used std.Node it's not liftable - need to consider what to do with it, the correct way is to use the builtin nodeof().

[mergify]

Thanks for contributing, @yoav-steinberg! This PR will now be added to the merge queue, or immediately merged if yoav/fixes_due_to_lift_unmuting is up-to-date with main and the queue is empty.

[eladb]

This is very cool