add MarshalSize() method to all packets

[aler9]

Description

This allows to get packet sizes without marshaling, a feature already provided by pion/rtp in case of RTP packets. It replaces a lot of functions that were implemented differently from packet to packet.

This is required in order to implement MarshalTo() and marshal packets into pre-allocated byte slices, greatly improving performance.

[codecov[bot]]

Codecov Report

Merging #128 (9245955) into master (677965a) will decrease coverage by 0.31%. The diff coverage is 86.84%.

@@            Coverage Diff             @@
##           master     #128      +/-   ##
==========================================
- Coverage   75.81%   75.49%   -0.32%     
==========================================
  Files          21       21              
  Lines        2398     2408      +10     
==========================================
  Hits         1818     1818              
- Misses        482      492      +10     
  Partials       98       98              

Flag	Coverage Δ
go	75.49% <86.84%> (-0.32%)	:arrow_down:
wasm	75.49% <86.84%> (-0.32%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
compound_packet.go	65.16% <0.00%> (-4.72%)	:arrow_down:
extended_report.go	67.74% <0.00%> (-0.64%)	:arrow_down:
packet.go	69.73% <ø> (ø)
raw_packet.go	43.47% <0.00%> (-4.15%)	:arrow_down:
receiver_estimated_maximum_bitrate.go	66.50% <ø> (ø)
full_intra_request.go	67.24% <100.00%> (ø)
goodbye.go	86.60% <100.00%> (ø)
picture_loss_indication.go	86.27% <100.00%> (ø)
rapid_resynchronization_request.go	78.26% <100.00%> (ø)
receiver_report.go	83.57% <100.00%> (ø)
... and 6 more

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