s/sexp_string_length/sexp_string_size/
Make sexp_string_ref/sexp_string_set Unicode aware.
Provide cursor level alternatives for efficiency.
Document all of this.
Documentation looks much nicer now (or at least more familiar).
Test cases become ugly because we need to double the escape in
strings. Also escaping requires \"\\" which in a Scheme string
gets written \\"\\\\". Consider \\ as a shortcut (which is still
\\\\ in a string).
