The current implementation of closure conversion creates closure environments for each
new scope. This change tries to minimize the number of closure environments, and thus
closure environment allocations, when possible by merging "adjacent" closure environments.

To merge two closure environments, they have to:

1. Be captured by exactly the same set of closures
2. Have no backwards branching between the allocation of the two environments
3. Have the same environment lifetime

If so, all of the variables will be merged into a single closure environment. 

Fixes #29965