Make writer consume z_bytes_t and then return it upon finishing all writes.

[DenisBiryukov91]

Describe the release item

Make writer take z_bytes_t by move, instead of by mutable reference.
Provide a method, taking writer by move and returning resulting bytes, once all writes are done.
This is to prevent creating several writers for a single z_bytes instance and also avoid side effects when cloning z_bytes.