Ensures that environments and synths can correctly be destroyed

[colinbdclark]

This pull request incorporates @waharnum's #184, and adds a unit test to show the fix, along with a few other scoping tweaks throughout the code base.

[waharnum]

I can confirm that using this branch addresses the issue I was seeing with the sonifier in https://github.com/waharnum/chartAuthoring/tree/GPII-1584