It turns out that we have some outputs (accumulators) for which
it only makes sense to have a `DependencyIndex` (they don't have
individual keys to identify).
We also track whether reset is required at the ingredient level.
For tracked struct fields, we were not using `push_mut`,
and I think that was an oversight.
The plan is to do a "dependent ingredient" interlinking pass
once the database is constructed.
I realized I can do this better:
* require that outputs are DatabaseKeyIndex, fewer unwraps,
more clearly justified
* when we validate result of tracked fn, also validate its outputs
(this is incompletely implemented, would ideally be separated
into its own commit, but I'm short for time)
The last step will allow us to keep the memoized results for
assigned values and means we don't have to eagerly clear them.
If we see an "assigned value" that is not verified in the current
revision, it can simply be considered dirty.
We can still delete them when entities are deleted, but they're
less special.
Rename QueryInputs to QueryEdges.
Modify its fields to track both inputs and outputs.
The size of the struct doesn't actually change,
as the separator comes out of padding.
Rename QueryInputs to QueryEdges and modify its fields
to track both inputs and outputs. The size of the struct
doesn't actually change, the separator comes out of padding.
