Previously we implemented update as a simple `Delete` + `Insert`
procedure which seemed okay for the moment but it wasn't. `Delete` can
trigger balance and a post balance `seek` which will leave cursor
pointing to an invalid page which `Insert` will try to insert to.
We solve this by removing `Delete` from the execution plan and rely on
`Insert` to properly overwrite the cell where the rowid is the same as
the one we are inserting.
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.
