Removing `String()` method from `Packet` interface

Per discussion on Slack, this patch undoes the addition
of a `String()` method on a Packet. Compound packets
now use the new `stringify()` function if a contained
Packet doesn't include a `String()` method.

Removes dumpBinary debug function

CI is complaining of deadcode.

Some fix about TransportLayerCC

Some fix about TransportLayerCC

Add bounds checking to unmarshal

Return error (instead of panic'ing) when we get a RTCP packet
with an invalid length

Add a Marshaller for an unvalidated packet slice

Relates to pion#29

Respond to linter

Relates to pion#25

Mass replace pions -> pion

Pions organization was renamed to pion

Relax validity check on Unmarshal

Leave full validity check on Marshal.

Relates to pion#18

Add some decoder tests

Relates to pion#13

Add REMB packet support

Used to send bitrate estimation.