[plt-scheme] hash-table reflection

It would be nice if I could tell what flags a hash-table was created
with (i.e. 'weak or 'equal).  Otherwise there's no way to implement a
general hash-table-copy.  I suppose I'd be happy if MzScheme included
its own hash-table-copy, although since what I really want is
deep-copy, I'd just have to go and replace all the values with copies
anyway, so doing the shallow copy first is probably a waste of time...

--dougo at ccs.neu.edu