WIP: simplify interface to TWCC 3/4

[jech]

This is part 3/4 of a multi-package series.

Part 1 is at #297
Part 2 is at pion/webrtc#3002

Please don't merge until the series is complete.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.

Project coverage is 71.03%. Comparing base (82bca40) to head (888fadd).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/cc/interceptor.go	0.00%	10 Missing ⚠️
registry.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #298      +/-   ##
==========================================
- Coverage   71.26%   71.03%   -0.24%     
==========================================
  Files          80       80              
  Lines        4476     4492      +16     
==========================================
+ Hits         3190     3191       +1     
- Misses       1153     1166      +13     
- Partials      133      135       +2     

Flag	Coverage Δ
go	71.03% <0.00%> (-0.10%)	⬇️
wasm	?

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