We get a speed up (~5%) and memory savings (~10%) on initial sync from
using a HashMap, though it's hard to tell precisely how much savings
because it's quite nonlinear.
I haven't tested de/serialization. Some work needs to be done there to
split up the UTXO set since it takes forever to saveload.
We were conflicting with the Rust stdlib trait Hash, which is used
by various datastructures which need a general hash. Also implement
Hash for Sha256dHash so that we can use bitcoin hashes as keys for
such data structures.
Andrew Poelstra