C-level string API redesign with cursor support.
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.
sexp_string_size and sexp_string_length both return sexp_uint_t.
Adding missing types.
Making C string port API names match the Scheme side.