[R6RS] modules

[William D Clinger]

> So scanning for implicit exports is indeed complex and potentially
> expensive, even for syntax-rules.

I am not convinced.  It isn't hard for a compiler to assume that
*all* of a module's variables are implicitly exported, and that's
a conservative approximation.  Scanning for implicit exports will
take very little time in most cases, and in the rare cases where
it threatens to become expensive, the static analyzer can give up
and widen to the *all* approximation.  Non-optimizing implmentations
can use the *all* approximation for all modules.

Will