Improve performance of json_encode()

Encoding with default settings is about 20-30% faster, encoding with JSON_UNESCAPED_UNICODE about 15% faster
Decoding is also ~5% faster

Encoding performance is achieved by avoiding the UTF-8 to UTF-16 conversation. The slight improvement to decoding performance is probably due to more inlining oppertunities as ``json_utf8_to_utf16`` is now only used when decoding.

Binary size increases by 56 bytes.