core: implement ssz for all objects with tests

[tcoratger]

Should close #11

[sauliusgrigaitis]

Grandine is also working on Lean client and we use our own SSZ and other libraries so best would be if cryptography library is not coupled with some particular client's library. Likely the solution is to ship SSZ related code as a separate crate.

[KolbyML]

For BLS on the Beacon chain what is normally is done, is the BLS crate has a from_bytes or to_bytes function, then a Beacon client would put those bytes into a FixedBytes then serialize it to ssz.

I am assuming that is what @sauliusgrigaitis wants. @tcoratger do you still want us/ream to test this PR, or should we wait for what @sauliusgrigaitis is requesting?

[KolbyML]

It works for us ReamLabs/ream#992