Make all the things immutable

[jeffreyguenther]

Because Shiro will try to make use of as much concurrency as makes sense, we should try to make as many things immutable as possible. Trust the JVM to clean up our mess and deallocate memory as it sees fit.

Enforcing immutability is an important principle for Shiro to follow. Any time the API is used to modify the current runtime state a new instance of Program should be created. This will allow us to represent histories of action in an efficient manner.