Numerous bugfixes:
1. Delta does not carry around the new environment any longer. It only has
a link to the environment in effect when it was first pushed on to the
value stack. Instead, the new environment is passed around as parameter
arguments in more functional style.
2. The control stack used by the CSE machine is a copy of the one
contained in delta bodies. Thus, delta stacks remain untouched in case the
delta needs to be reused.