bug(rln): Type mismatch in witness json serialisation and deserialisation functions

[seemenkina]

The serialisation and deserialisation functions have problems with json field types because all value are serialized as string and deserialize required number value. And test function cover only deserialisation on constant value

Serialisation to String:

zerokit/rln/src/protocol.rs

Lines 775 to 777 in fe2b224

	for v in rln_witness.path_elements.iter() {
	path_elements.push(to_bigint(v)?.to_str_radix(10));
	}

Deserialisation from u64 :

zerokit/rln/src/protocol.rs

Lines 247 to 248 in fe2b224

	for v in identity_path_index_array {
	identity_path_index.push(v.as_u64().ok_or(Report::msg("not a u64 value"))? as u8);

[rymnc]

addressed in #253