Added a `write_buf` function to write one chunk of data directly

[gavincangan]

Since the buffer size can be overridden, this could help cases where someone needs to send a small chunk of data without breaking it up across multiple messages. I needed this to reduce latency

[gavincangan]

Btw I'm not sure if should perhaps call it write, since there's an overloaded write(uint8_t* buf, size_t len) function in the original Serial class