This makes tenv and lenv, which were arguments to almost everything,
into parameters. The only time they need to be set is after parsing
and before calling anything else, so there's one use in compile/defns
and one use in compile/interaction.
While I was at it, I took a lot of the infrequently-changing
arguments to typecheck-expression and made some typechecker-local
parameters also.
svn: r326