* core: partially implement sumtree; appending and pruning work
* core: add simple merkle-root-sum-calculating function.
* core: implement replacement in sumtree
* core: implement de/serialization for sumtree
* core: make sumtree index use hashes rather than data
* core: prevent double-adds to sumtrees
* core: replace sumtree's separate sums with a Summable trait
* core: make SumTree::contains() return the index if an element exists
* core: cleanup slicing impls for Hash
* core: clean up Readable trait, implement Readable/Writeable for various integers
* core: change Hash debug output to hex
* core: correct warnings in all modules
* Replace AsFixedBytes with Sized + AsRef<[u8]>
* Add AsRef<u8> to impl_array_newtype!
* Include AsFixedBytes as marker trait
* Related fixes
* Remove Deref
* use MESSAGE_SIZE constant instead of 32
* use Hashed trait in genesis
* safer serialization of Hash
* Hashed of [u8] should send only data to hash function, not length + data
* Safer serialization of Proof
* Safer serialization of Commitment
* Safer serialization of RangeProof
* introduce read_limited_vec instead of potential panic in conversion
