Simplified version of Helmut Eller's autodoc rewrite

This is the original commentary:

"Cleaned up and simplified. I think most of functionality was
perserved.  There was a "multiline cache" feature which I was not
able to understand--despite my best efforts.  So I deleted that."

(cherry picked from SLIME commit
930f364853e518419978082c9a8455071e537b59)

Conflicts:
	contrib/ChangeLog
	contrib/slime-autodoc.el
	contrib/test/sly-autodoc-tests.el