Revert "Generify scopes to Object, in preparation for using Object in"

This reverts commit a57d94a4c02063e2df2c655fbe349d2b4769ba41.

This change is right long term but breaks the current compiler in
many unpleasant ways. Rather than sort them out, I'm reverting
this for now. Once we switch to IR+JIT we should be able to
reinstate this commit.