Import merkletree package

wow that +/- ratio.

The merkletree library isn't a perfect fit for our purposes; I had to modify it to use uint64 for proofIndex and numSegments. I'm also not a fan of how the base segment is lumped in with the hash set; that caused me a bit of confusion.

I wound up implementing (in the crypto package) a Sia-specific tree type so that you can do things like PushObject (and Root returns a crypto.Hash instead of a []byte). I'm pleased with how this cleaned up the code. I could take it a step further by making PushObject variadic; then you could just call it once with a bunch of objects. In practice this is only useful in one function though (consensusSetHash).