EQUALP hash tables (and more ...)

[David A. Moon <Moon@STONY-BROOK.SCRC.Symbolics.COM>]

DCP's message of 12 May said most of what I want to say on the subject of
generic hash tables, Symbolics' experience with them, and what issues are
involved in implementing them.

One other point: I suspect that rather than specifying the equivalence
function and the hashing function as separate arguments to
MAKE-HASH-TABLE, you would be better off introducing a DEFHASH special
form that defines a new flavor [sic] of hash table.  That way, you
package up the equivalence function and the hashing function together
into something that has a name, and you don't have to face the issue of
inconsistent combinations.  Furthermore, the DEFHASH for EQ and EQL hash
tables has already been done by the Common Lisp implementation, so you
don't have to publish the name of their implementation-dependent hashing
function.

Our CL implementation already uses the :TEST argument to MAKE-HASH-TABLE
as a key looked up in a table of flavors of hash table, but we haven't
told users how to add their own entries to that table.