Add cooperative REPL server module.

Add cooperative REPL server module.

Modified-by: Mark H Weaver <mhw@netris.org>
* module/system/repl/coop-server.scm: New module.

* module/system/repl/repl.scm (start-repl): Extract body to start-repl*.
  (start-repl*): New procedure.
  (run-repl): Extract body to run-repl*.
  (run-repl*): New procedure.

* module/system/repl/server.scm (run-server): Extract body to
  run-server*.
  (run-server*): New procedure.

* doc/ref/api-evaluation.texi (Cooperative REPL Servers): New node.

* module/Makefile.am (SYSTEM_SOURCES): Add system/repl/coop-server.scm.