Improved hash table package for Scheme48

This library for Scheme48 implements much more rich hash tables than
what Scheme48's built-in hash tables provide, notably object identity
tables (with arbitrary objects for keys), tables with GC-sensitive hash
functions in general, and head- & tail-weak hash tables.

Taylor Campbell wrote this code and this associated documentation; he
places it in the public domain.

The scheme/ directory contains the Scheme source code. Load the file
scheme/packages.scm for the definitions of the relevant interfaces &
structures. The RIATABLES structure exports everything relevant to the
package.

There is currently no extended documentation.  Use the source, Luke.
This may change soon.

Send bugs, comments, questions, or anything to Taylor Campbell via
email to campbell@bloodandcoffee.net or via IRC on irc.freenode.net,
where he can be found in the channels #scheme & #scsh as Riastradh.