Complete rewrite of URL support. New design implements generic codec

Complete rewrite of URL support.  New design implements generic codec
support for URIs as defined in RFC 2396, which is both more general
and easier to use than the old design.

All names have been changed to use the string "uri" rather than "url".
A minimal number of URL procedures has been retained to support IMAIL
until it is rewritten to use the new design.

The package has been renamed to '(runtime uri).