functional hash operations

[Paul Jarc]

Sometimes it would be useful to have functional operations on hashes:
"make a copy of this hash which also has this additional entry", etc.
It would also be nice to make such copies share as much memory as
possible with the original hash.  This is easy enough to do by taking
advantage of the knowledge of the representation of hashes as vectors
of alists, but is it safe to assume that representation?  Or might the
representation change in the future?


paul