Somewhat related to #4

It would be nice if RtpPacketBuilder would be generic over the payload type (this can be added in a backwards-compatible way by making it RtpPacketBuilder<T = &'a [u8]>), and a custom callback for writing out the data could be provided to write_into().

This way one could provide a custom type for the payload and "collect" chunks as they're written out to avoid copies. Header/padding would still have to be passed to that function as a &[u8], but actual payload could be taken over more cheaply by e.g. using bytes::Bytes or a gst::Memory and then collecting the packet in multiple chunks that are then sent over the network via sendmsg().

RtpPacketBuilder::write_into() requires a mutable slice. Unfortunately, a mutable slice always has to be initialized completely, which is suboptimal here.

Generally, some API around Writer might be nice to have.

Could make it some kind of RtpPacketMut (better name please) that derefs into the normal RtpPacket for reading purposes, and in addition to the fixed headers it could also give mutable access to specific header extension bytes and the CSRC array (so e.g. CSRCs can be changed but not added/removed).