Parse a list of one or more parameter declarations.
(parse-parameter-declaration-list parstate) → (mv erp paramdecls ellipsis span new-parstate)
We parse the first parameter declaration, which must be present. Then if there is a comma there may be another parameter declaration, but not necessarily, because we may have an ellipsis instead. So we must read a bit further to check that; if there may be indeed another parameter declaration, we recursively parse the remaining list of one or more.