[plt-scheme] dictionaries

In SVN, a new `scheme/dict' library support generic operations on
"dictionaries": `dict-ref', `dict-set!', `dict-set', etc.

Examples of dictionaries include hash tables, assoc lists, and vectors.
The `prop:dict' property lets a programmer create new dictionary
implementations that work with the generic operations.

The library also provides `make-custom-hash', which creates a
dictionary (that is implemented in terms of hash tables) given equality
and hashing procedures.


Matthew