Some further SSZ work

[Nashatyrev]

As the follow up to PR #115 I would like to do the following things:

• add Incremental hashing of basic lists (packed)
• refactor SSZBasicAccessors
• make ssz module independent of others
• do some performance benchs and probably pass some buffer for serialization to avoid creation of numerous BytesValues objects
• additional performance gain can be obtained by optimized serializing of BytesValue or byte[] SSZ fields

[zilm13]

make ssz module independent of others

@Nashatyrev
It currently depends on util and types. What's wrong with such kind of dependency?