Reduce toplevel environment decls and allow them in user config

Reduce toplevel environment decls and allow them in user config

The root of the dynamic environment is passed separately to Eval.exec'
to allow user config to re-declare dynamics (like regular vals). This
uncovered (and perpetuates) a bug with process DVal/DEnv:

  val foo = "foo";
  val bar = foo;
  val foo = "bar";

When bar is expanded, it now has the value "bar" instead of
"foo", which is wrong.