perf(reactivity): better computed tracking

[jods4]

I made computed use track and trigger directly.

Why?

• Code is shorter and simpler. trackChildRun depended on activeEffect and was basically a copy of tracking internals. The new code is straightforward to understand: it simply calls track when value is read.
• Debugging is more clear. If you look at dependencies during debugging, you'll see the computed, rather than what's cached behind.
• It's more efficient. If the getter depends on hundreds of values (typical use case: computed filters or summarizes an array), those 100+ dependencies will be registered on the computed effect, but then transitively copied as dependencies of any effect that reads the computed. With N dependencies and M readers, before PR we had N*(M+1) entries, after PR N+M.
  Not only does it save space, it also saves time if many dependencies change at once. If N dependencies change at once, the number function calls is the same as the entries above (after PR, computed only calls trigger once, rather than having a notification for every dependency that changes).

[yyx990803]

👍 This is indeed better, thanks!