Substitute json_string with SCJsonString custom function.
SCJsonString will ensure string passed is json-encodable (utf-8).
If it's not, the string will be converted in such a way that any
non-printable character will be encoded in its hex form.
The resulting json object will be returned.
rust modification will encode any non-printable character during its
conversion in to_cstring.
Rust strings are UTF8 and we cannot yet rely on jansson
having json_stringn on all supported OS distributions yet
so sanitize strings to ascii before printing.
Also add set_string_from_bytes which is like set_string, but
accepts a byte array as input.