[plt-scheme] a little more keyword-argument support

In SVN, the `class', `send', etc. forms now support methods with
keyword arguments.

The `apply' function now accepts arbitrary keyword argument, and it
passes them on to its argument function. Similarly, `keyword-apply'
accepts direct keyword arguments to be merged into the supplied lists
of keys and values.

For fine-grained control over keyword procedures, a new
`procedure-reduce-keyword-arity' function supports refining the set of
keywords that are accepted/required by a procedure --- especially one
created by `make-keyword-procedure'.


Matthew