Execute a set of instructions in the context of an H2O cloud.
An Env (environment) object is a classic stack of values used during
execution of an AstRoot. The stack is hidden in the normal Java execution
stack and is not explicit.
For efficiency, reference counting is employed to recycle objects already
in use rather than creating copies upon copies (a la R). When a Frame is
`pushed` on to the stack, its reference count is incremented by 1. When a
Frame is `popped` off of the stack, its reference count is decremented by
1. When the reference count is 0, the Env instance will dispose of the
object. All objects live and die by the Env's that create them. That
means that any object not created by an Env instance shalt not be
Therefore, the Env class is a stack of values + an API for reference counting.