JSON serialization/deserialization of bytes is incorrect

Currently, we use our own serialization of bytes in json.hpp but we use picojson for deserialization.

We've observed that for some bytes the serialization results in a string that is incorrectly decoded by picojson.

Example:

String = ""\"\\/\b\f\n\r\t\x00\x19 !#[]\x7F\xFF"

Result of our own encoding: "\"\\\"\\\\\\/\\b\\f\\n\\r\\t\\u0000
u0019 !#[]\\u007f\xFF\""

picojson's encoding: "\"\\\"\\\\\\/\\b\\f\\n\\r\\t\\u0000
u0019 !#[]\\u007F\\u00FF\""

Fix:
We just use picojson to serialize bytes for consistency.