Add #!fold-case directive

[jcubic]

I started working on #!fold-case and #!no-fold-case. But it requires having real ports as user input to allow changing the state in the REPL.

This works:

(with-input-from-string "#!fold-case (FOO BAR BAZ)" read-all)
;; ==> ((foo bar baz))

Maybe the REPL should create a single instance of Parser and use exec on the result.