sexp.lisp: Follow *print-case*

Much of the serialization functions output hard-coded strings such as "(:SEQUENCE ", etc.

When `*print-case*` is set to `:downcase` or `:capitalize`, the result is a mix of difference cases.

It would be nice if all hard-coded strings would be cased according to `*print-case*`.

Look at sexp.lisp for serialization functions that output hard-coded strings like '(:SEQUENCE '. Understand how *print-case* affects symbol printing in Common Lisp. Modify these strings to respect *print-case* settings, ensuring consistent casing. Test by setting *print-case* to :downcase or :capitalize and verifying output matches.

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