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.
This adds an interrupt() method to Statement that allows apps to
interrupt a running statement. Please note that this is different from
`sqlite3_interrupt()` which interrupts all ongoing operations in a
database. Although we want to support that too, per statement interrupt
is much more useful to apps.
