rename LET_SEQUENTIAL to LETSEQ
Let Environment self-eval
Let Transformer self-eval too
Fix syntax_test reference to old LETSEQ spelling
Make procedure generation capture a child of the current environment
Add an interface for push/popping child frames of environments
Set active environment frame during Procedure extent

So, actually, we didn't want an Environment frame stack.
Just an active environment which can be set at any time
for some duration of the program. Frames are owned by
procedure continuations rather than by their parent
environment.

I think this makes sense. I think.