Allow making raw calls returning a value.

[clesaege]

Currently the function for making raw calls is call, however the documentation (p42 https://media.readthedocs.org/pdf/solidity/develop/solidity.pdf) specifies that

It is not possible to access the actual data returned (for this we would need to know the encoding and size
in advance).

It would be nice to have a way to make a raw call returning the raw return value. Or to have a raw call where the return value can be specified.

It should work like:
var (x, y) = callReturn[uint, string](bytes4(keccak256("fun(uint256)")), 5778));

That's a real need, in my case for action callback kleros/kleros-interaction#13.

[axic]

For callbacks I suggest to have a look at http://solidity.readthedocs.io/en/develop/types.html?#function-types.

Regarding returning data please see #434. And for ABI decoding see #1707.

[clesaege]

Thanks.
I was aware of #1707 but not of #434. I'll move the discussion to #434.