Move result row to `ProgramState` to mimic what SQLite does where `Vdbe`
struct has a `pResultRow` member. This makes it easier to deal with result
lifetime, but more importantly, eventually lazily parse values at the edges of
the API.
The name "row result" is confusing because it really *is* a result from
a step() call. The only difference is how a row is represented as we
return from VDBE or from a statement.
Therefore, rename RowResult to StepResult.
