ReactiveX / rxjs / 8041

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perf(Subscription): improve parent management (#4526)

Merge the `_parent` and `_parents` fields on the `Subscription` class
into a single `_parentOrParents` field, which can hold either `null`
(in case of no parents), a `Subscription` instance (in case of a
single parent), or an array of `Subscription`s (in case of multiple
parents). This not only shrinks the size of `Subscription` (and
subclass) instances, but more importantly reduces the number of
megamorphic property access sites in the hot `Subscription` methods,
especially in `unsubscribe()` and `add()`.

Also inline the `Subscription#_addParent()` method into
`Subscription#add()`, as it's the only call site anyways and
this removes another hot megamorphic `LoadIC` that was only needed
to lookup `subscription._addParent` in the call site.

Finally remove the `hasErrors` variable from `unsubscribe()` method
and instead just check `errors` variable directly.

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