Marshal should not have extension after delete

Do not validate non spec obu-size

Fix H265Payloader (#350)

Add H266 de/packetizer (#347)

* Add H266 de/packetizer

Add MarshalTo for zero-allocation extension marshalling (#344)

Add Marshal benchmarks for RTP extensions

Add benchmarks for Marshal method on:
- AbsSendTimeExtension
- AbsCaptureTimeExtension (with and without offset)
- AudioLevelExtension
- TransportCCExtension
- PlayoutDelayExtension
- VLA

Use atomic instead of lock in sequencer

Add SetExtensionWithProfile

Allows for conversion between RTP Header Extension Profiles easily.
'One Byte' packets can be updated without doing a full re-creating
of the Extension Headers.

Resolves #255
Resolves #249

Improve Header Unmarshal performance

No need to alloc empty slice if CSRC=0

Few fixes for Header.Unmarshal

- Skip one-byte padding with non-zero length as specified in RFC;
- Return proper header length when one-byte extension with ID=15 or
  one-byte padding with non-zero length was found;
- fixed check for end of extended header payloads;

Fix unmarshalling extension-only packet

Unmarshalling packets with extensions without RTP payload caused
'RTP header size insufficient for extension' error.
Fix data size check condition.