Closes #9: New output respects existing REPL results or presentations.

* contrib/slime-presentations.el (slime-presentation-write-result): Idem,
but for presentation results.

* contrib/slime-repl.el (slime-repl-emit-result): After emitting a result,
        update `slime-output-end' marker.

* contrib/test/slime-repl-tests.el (mixed-output-and-results): New test.
When enabling a contrib, dependencies are also enabled recursively.

Without this, setting up or testing a contrib dependending on others
often fails. Assumes enable functions are quick and idempotent.

* slime.el (define-slime-contrib): Call enable functions for
dependencies recursively. (slime-contrib-all-dependencies):
Renamed from `slime-contrib-all-dependencies'

* contrib/Makefile (check-%): Remove redundant `slime-setup' call and call
`slime-contrib-all-dependencies'.