Restrict values that can be assigned to compile-time variables

[GearsDatapacks]

At the moment, you can assign any expression to a compile-time variable. This allows for some weird behaviour:

&a = something(1)
something(2)
do_something_with(&a)

Here, &a refers to the return storage of the something function, but since that function has been called since we assigned to it, the value is now different to what it would have been if accessed earlier. The above code doesn't make much sense to be valid, so we should probably disallow it. There are a few such similar example which we need to decide how to handle.