Replace `fixed` by standard `encode` function.
Replace `fixed` by standard `encode` function.

This way, we can use
```
BOOST_CHECK(callContractFunction("t(fixed8x128)", make_pair(rational(2, 3), 128)) == encodeArgs(u256(9)));
```
which is a bit more self-explanatory, I would say.
ruchevits/solidity,ruchevits/solidity,ruchevits/solidity,ruchevits/solidity