OwnedValue has become a powerhouse of madness, mainly because I decided
to do it like that when I first introduced AggContext. I decided it was
enough and I introduced a `Register` struct that contains `OwnedValue`,
`Record` and `Aggregation`, this way we don't use `OwnedValue` for
everything make everyone's life harder.
This is the next step towards making ImmutableRecords the default
because I want to remove unnecessary allocations. Right now we clone
OwnedValues when we generate a record more than needed.
We currently have two value types, `Value` and `OwnedValue`. The
original thinking was that `Value` is external type and `OwnedValue` is
internal type. However, this just results in unnecessary transformation
between the types as data crosses the Limbo library boundary.
Let's just follow SQLite here and consolidate on a single value type
(where `sqlite3_value` is just an alias for the internal `Mem` type).
The way this will eventually work is that we can have bunch of
pre-allocated `OwnedValue` objects in `ProgramState` and basically
return a reference to them all the way to the application itself, which
extracts the actual value.
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.
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.
