Encoder::buffer_filled and ::len_filled

[jgraef]

This PR adds two methods to the Encoder:

• buffer_filled: returns only the portion of the buffer that was written to, i.e. the serialized message.
• len_filled: returns the number of bytes that have been written.

In my usecase I'm reusing a buffer for sending/receiving. I don't want to truncate the buffer to length 0, just to have to grow (and thus initialize) it later again. Encoder writes to the buffer from the beginning anyway and only grows it as needed. If the buffer was larger that the serialized message, the buffer() method will still return the whole buffer, including any garbage at the end of the buffer.

With buffer_filled and len_filled you can easily get a slice of the serialized message.

[stappersg]

There only a commit summary, please extend the commit message.
(FYI: I'm not project lead, just some random Joe how cares about this project.)

[leshow]

lgtm, thanks!