perf(Subscription): improve parent management

[bmeurer]

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 Subscriptions (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.

[cartant]

LGTM

[coveralls]

Pull Request Test Coverage Report for Build 8037

• 28 of 29 (96.55%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 96.805%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/internal/Subscription.ts	25	26	96.15%

Totals
Change from base Build 8025:	0.05%
Covered Lines:	5242
Relevant Lines:	5415

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💛 - Coveralls

[benlesh]

Thanks again, @bmeurer! I really appreciate the work you're doing.

[bmeurer]

Thanks 👍