This code was unmaintained, is unlikely to work on the majority of systems
(since it holds the whole utxoset in RAM, and not in a terribly efficient
manner), and has a dependency on `eventual` which has been broken for a
long time.
The library no longer compiles on nightly because of this, and without any
known usecases for `UtxoSet`, nor good ability to test it, I'm simply
removing the code.
I recommend anyone who cares about this extracts the code from the previous
commit and creates a new crate. It should be more featureful anyway, e.g.
support a backing store.
Andrew Poelstra