To build libraries that use send/suspend, the library code must reside inside
a unit that imports send/suspend.  Servlets that use the library must
link the main servlet with the code from the library using a compound-unit.

helper.ss
  This file provides the function _get-number_ that uses send/suspend
  to interact with the consumer.
  
helper-sig.ss
  This file provides a unit signature required for linking to the helper
  library.

add.ss
  The add servlet links to the helper unit and calls get-number.

multiply.ss
  The multiply servlet also uses the helper library.