Establish a contract around Writer::write()

[aliddell]

In a previous PR we altered bool Writer::write(const uint8_t*, size_t) to size_t Writer::write(const uint8_t*, size_t), i.e., to return the number of bytes written

In this PR we establish the following contract:

Let uint8_t* data be a buffer of size nbytes_in, and let nbytes_out be the value returned by Writer::write(data, nbytes_in). If nbytes_out < nbytes_in, then Writer::write(data + nbytes_out, nbytes_in - nbytes_out) should do what we expect. In other words, when writing the 2nd (or 3rd, or ...) part of a partial frame, the Writer should copy the data to the correct position in its buffers, instead of overwriting old data or beginning a new frame.