address review comments

- Packet::set_len: panic instead of silently clapping the value
- Packet::copy_from_slice: panic instead of silently truncating
- rename some internal constants related to the size of the parts of Packet
- doc tweak: Packet is PHR + PSDU
- keep Channel enum as it is (removed TODO)
- add send_no_cca method
- shortcut ccaidle to txen in try_send
- simplify transitions to the TXIDLE / RXIDLE state

Author: japaric