Remove duplication in STHash by using generics.

* STHash256, STHash160, STHash128 replaced by STHash<256>, STHash<160>, STHash<128>. 
* STUInt8, STUInt16, STUInt32, STUInt64 are now instances of the same generic class.
* New generic accessors for Serializer.
* Net hundreds of near-duplicate lines of code deleted.