Implement `UTF8JsonGenerator.writeRawValue(SerializableString)` (and `writeRaw(..)`) more efficiently

[cowtowncoder]

(note: follow-up for #480)

Current implementation is:

    public void writeRawValue(SerializableString text) throws IOException {
        _verifyValueWrite(WRITE_RAW);
        byte[] raw = text.asUnquotedUTF8();
        if (raw.length > 0) {
            _writeBytes(raw);
        }
    }

but latter part should be implemented as something like:

        int len = name.appendUnquotedUTF8(_outputBuffer, _outputTail);
        if (len < 0) {
            _writeBytes(name.asUnquotedUTF8());
        } else {
            _outputTail += len;
        }