perf(computed): avoid updates when source signal value is restored

[johnsoncodehk]

This PR relies on changes from #14.

Consider the following situation:

const s = signal(false);
const c = computed(() => s.get());
c.get();
s.set(true);
s.set(false);
c.get(); // <-- should not recalculate

In the last line c.get() should not cause computed to perform the getter because the value of s is restored.

[johnsoncodehk]

Close due to #19 changes, the solution is no longer feasible.