(scheme-mode-syntax-table): Mark ; as being also the

(scheme-mode-syntax-table): Mark ; as being also the
second char of a comment-start sequence.
(scheme-sexp-comment-syntax-table): New var.
(lambda, define): Set their scheme-doc-string-elt property.
(scheme-font-lock-syntactic-face-function): Handle sexp-comments.
Use lisp-font-lock-syntactic-face-function now that it properly
handles |...| symbols.
(scheme-mode-variables): Set lisp-doc-string-elt-property,
parse-sexp-lookup-properties and font-lock-extra-managed-props.