Previously we were ignoring if the monero wallet rpc was not found and
unpacked from archive leading to a failure down the line when trying to
run a non-existent executable. Bail when the executable is no found in
the archive.
If the monero wallet rpc has not already been downloaded we download the monero cli package and extract the wallet rpc. The unneeded files are cleaned up. The monero wallet rpc is started on a random port which is provided to the swap cli.
We added a fork of tokio-tar via a git subtree because we needed a tokio-tar version that was compatible with tokio 1.0. Remove this subtree in favor of a regular cargo dependency when this PR merges: https://github.com/vorot93/tokio-tar/pull/3.
