this patch refactors the MAC PDU class, the main changes are:
* add to_string() method to pretty print PDU and subheaders
This allows to have a single log entry per MAC PDU with all its
contents.
It removes the C-style fprint() method
* Simplity payload vs. w_payload_ce
Before we've used payload when reading the PDU and w_payload_ce
as a buffer when writing. In all getters we needed to differentiate
between both. Now payload points to w_payload_ce initially and is
only updated when parsing a new PDU.
* add various helpers, e.g. to get subheader index, update a BSR, ..
* fix PDU test to use new to_string() method
Andre Puschmann
Francisco Paisana