At the moment `Ap` state contains vector of generations, although the ap instruction itself allow to specify only one stream. It was done to support several streams in a result position in a future. But the current realization allows to handle empty vector that was implemented to support states for scalars and it turned out (for more info see #326) that states for scalars aren't actually needed.
Closes#355.
Many internal structures refer to trace positions; this is an important type of values. In the code, it is sometimes `u32`, sometimes `usize`. While such variables usually have "_pos" suffix, having a dedicated type does improve code with type guarantees.
