[E-Lang] scope rules

[Jonathan S. Shapiro]

> In March, Ralph Hartley complained about the awkwardness of mutual
> recursion in E, and advocated a different scope rule where definitions
> are in scope starting from the `{' they're immediately nested within.
> Mark Miller answered
>
> A technical objection: what do we do at the interactive toplevel

The Scheme community spend a number of years on this question, and
ultimately came to a satisfactory resolution. If we adopt this proposal, we
should do what they did rather than re-execute their careful analysis of the
semantics.