support slightly more CLHS references

* contrib/slime-references.el (slime-references-properties): Support
`:type' and `:system-class' `:ansi-cl' references.
fix references to SBCL manual

I don't even want to think about how long it's been since SBCL switch
to an all-in-one-page manual format.

* slime-references.el (slime-lookup-reference-at-point): change format
for lookup in SBCL manual to match current all-in-one-page
publication.