do some more environment pruning

if the body of a let does not need a prefix of the outer environment,
remove that prefix before creating the environment that the let
produces.

could be extended if some parts of the rhss don't need the full
environment either.
min(list) isn't rpython
Merge branch 'master' into env-pruning
Merge branch 'env-pruning'

Conflicts:
	pycket/interpreter.py