add RTCRtpTransceiver

[fippo]

from #43. Not used yet.

[codecov-io]

Codecov Report

Merging #125 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #125      +/-   ##
=========================================
+ Coverage    99.8%   99.8%   +<.01%     
=========================================
  Files           6       7       +1     
  Lines        1020    1033      +13     
=========================================
+ Hits         1018    1031      +13     
  Misses          2       2

Impacted Files	Coverage Δ
rtcrtptransceiver.js	100% <100%> (ø)

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[fippo]

hbos' tests in wpt are probably quite good for testing this.