EEI: calls and return data

[axic]

I think since Byzantium has finally introduced returndatacopy and returndatasize the original result buffers of call* are obsolete (create is a different beast).

It would clean semantics and implementations a lot of if remove the result buffer support from calls and require clients to use returndata*.

Related #12

[axic]

cc @chfast @jakelang

[chfast]

I think it is good idea.