perf(@angular-devkit/core): avoid RxJS performance penalty in sync fs calls

[JoostK]

While testing compiler performance with ng9-aot-build-times I noticed RxJS's SafeSubscriber to show up as hot function. I opened ReactiveX/rxjs#5646 to mitigate it, however this change cannot be accepted because it breaks a certain usage pattern. As an alternative, this PR avoids the performance penalty by switching over the anonymous observer object to use 3 separate callbacks instead, such that RxJS doesn't have to create calling context objects using slow Object.create.

On a new CLI app, local testing indicates that this saves about 120ms or 2%, whereas the ng9-aot-build-times repo shows approximately 2 second improvement, or ~6%.

[alan-agius4]

@JoostK, can you change the commit msg type to fix? perf is not a valid type in the CLI repo

[alan-agius4]

LGTM, thanks

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