[R6RS] case-lambda definition

[R. Kent Dybvig]

The definition of case-lambda disallows case-lambda forms without any
clauses.  Although probably nonsensical in human-written code, I believe
this is unnecessarily restrictive for macros that generate case-lambda
forms, so I propose that we lift the restriction.

The change is easy to make both in the text and in the (new) macro
definition.

If there are no objections, I will ask for a vote Wednesday.  If anyone
prefers that I make this a formal comment I will do so.

Kent