If a query observes an untracked read, it gets changed_at equal to the
current revision. When we re-validate the query later, if it doesn't
do an untracked read this time, it gets changed_at equal to the
maximum of the dependencies. Crucially, this new changed_at may
be **older** then the previous value of changed_at. That is, we break
the rule that `changed_at` monotonically increases.
This can lead to missed re-executions down the line (see the added
test).
closes#66
We used to ignore constant inputs entirely. We now track them, but if we
find that a value is constant, we discard ITS inputs. This means that
-- if we track dependencies -- we have an "outer rim" of constant
values.
Also take the opportunity to reshuffle how derived inputs represent
their state.
