Remove scope abstraction that was only necessary for the legacy backend

[kjvalencik]

• Active check is no longer necessary since changing execute_scoped and compute_scoped to take &mut self
• Remove scope metadata since compute_scoped and execute_scoped are the only places scope needs to be tracked
• Apply simplifications everywhere from the first two changes

In order to use Drop for scopes--which is a little cleaner and protects us from panic leaks--env is stored both in the scope and on the Context. This additional pointer is a small cost and overall much smaller than the removed ScopeMetadata.

[dherman]

🚢