Bit shift operations

[jjcnn]

There is now a use case for left and right bit shifts: https://github.com/AragonBlack/fundraising/blob/master/apps/bancor-formula/contracts/BancorFormula.sol

Implementing shift operations in the standard library using existing Scilla features is pretty tricky, because we don't have operations that manipulate byte strings at the granularity of individual bits. It therefore seems better to add shift operations as builtins.

There's a design choice to be made regarding the type of these builtins, but we might be able to leverage the polymorphic builtin feature that I implemented, and which we then discovered we didn't need after all...