move churn modules into this repository

Obsoletes the `uproxy-churn` repository:
https://github.com/uProxy/uproxy/issues/445

This mostly just copies the code wholesale but I took the opportunity to rename the modules a little (it's more important they have descriptive names when there's a bunch living side by side):
* `turn` and `net` become `turn-frontend` and `turn-backend`, respectively
* `pipe` becomes `churn-pipe`

Unit tests pass and I tested all the sample apps manually.

Fewer repositories for all!
Great point. At some point we realised that we could just send strings between Freedom modules and this is how the nice copypaste sample app in uproxy-lib does it. So, I just changed the copypaste chat samples in this repo to do likewise, eliminating these files. Thanks!