This disables building the lightning-net-tokio crate on 1.34.2 on
Travis in anticiption of relying on async/await for
lightning-net-tokio, with the nice side-effect of reducing the
amount of work done on our slowest Travis job.
This also adds 1.39.0 to Travis as that is the new MSRV for
lightning-net-tokio and uses that for codecov generation.
This also moves fuzzing to Rust stable, as honggfuzz broke its MSRV
and it seems likely it will again in the future.
This is still pretty raw and ist mostly just a straight move from
rust-lightning-bitcoinrpc, but first step is to get it there,
second is to actually make it actually safe to use.