[plt-scheme] Web server library change

The following changes affect the 299 Web Server.

You no longer need to require `servlet-helpers.ss' to use the following procedures in your servlet:

 * extract-binding/single
 * extract-bindings
 * exists-binding?
 * extract-user-pass
 * build-suspender
 * make-html-response/incremental
 * report-errors-to-browser
 * redirect-to
 * permanently
 * temporarily
 * see-other
 * let*-bindings

Instead, use the following:

For module servlets:

 (require (lib "servlet.ss" "web-server"))

For unit/sig servlets:

 (require (lib "servlet-sig.ss" "web-server"))

The rationale: `servlet-helpers.ss' was required for nearly every servlet, in addition to either `servlet.ss' or `servlet-sig.ss'. Now they are combined, making the "Hello, world" servlet simpler to write, and reducing the number of libraries to memorize just to write a servlet.

-- 
Mike Burns