Write, review, deploy the basket2 pluggable transport.

I've been working on a successor to obfs4 called basket2, which combines a bunch of ideas from obfs4 and basket into one unholy abomination.

This ticket is the meta ticket to track development and initial deployment of the transport.

Code: https://git.schwanenlied.me/yawning/basket2

Deployment requires: legacy/trac#18333 (moved) along with updating some of the dependencies pulled in for obfs4.