Hi all,

I've attached record type printers for SRFI 45 promises and SRFI 41
streams. I've tried to make promise-visit more self-documenting with
the use of keyword arguments; let me know if you think that's an
improvement!

Also as discussed with Mark H Weaver, I've currently implemented the
format for promises as #<promise => ...> for unevaluated promises, and
#<promise = ...> for evaluated ones. Hopefully this is is easy to read
and will clearly distinguish between the two types, and still look
different from core promises too.

Comments are most welcome!

Thanks,
Chris.