Rename Serializable::hash() to Serializable::bitcoin_hash()

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.
Replace PatriciaTree with HashMap for UTXO set

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.