with-foreign-pointer-as-string macro is similar to
with-foreign-pointer except that var is used as the
returned value of an implicit
progn around body, after
being converted to a Lisp string using the provided args.
CFFI> (with-foreign-pointer-as-string (str 6 str-size :encoding :ascii) (lisp-string-to-foreign "Hello, foreign world!" str str-size)) => "Hello"