Implementing SSZ offset based variable size elements serialization

[zilm13]

Resolving #124

Regarding removed BytesValue concat/wrap optimization:
current optimization causes StackOverflowError on big lists, say, of 8192 elements (we have such lists in randao mixes, for example). At first I've tried to solve StackOverflow with 10c2543 but ongoing measurement shows that this optimization is not working anymore, at least for current SSZ implementation:

ModelsSerializeTest.beaconStateTest() running time:
MAX_GENERATION = 8 - 64 seconds
MAX_GENERATION = 32 - 370 seconds
copy everytime, no optimization -  2 seconds

so I've removed it finally. I guess, rebuilt SSZ need some new optimization at different view.

[mkalinin]

LGTM 👍