Attached is a patch to implement the changes detailed in
http://srfi-email.schemers.org/srfi-41/msg/3066997, which improves
the performance of `stream-constant` compared to the reference
implementation.

It probably doesn't improve performance against the Guile version
(which is already O(1) per element), but it's a cleaner implementation
that removes the need for the `list?` shadowing hack in `list->stream`.

This also removes support for circular lists for `list->stream` (which
was never supported in the reference implementation, and was added as
an implementation detail of `stream-constant` (which I originally
implemented using circular lists).

Comments welcome.

Cheers,
Chris.