ReactiveX / rxjs / 3753

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feat(distinct): remove `distinctKey`,  `distinct` signature change and perf improvements (#2049)

* feat(distinct): remove `distinctKey`,  `distinct` signature change and perf improvements

- Adds a limited Set ponyfill for runtimes that do not support `Set`
- `distinct` now supports an optional keySelector argument that the user can use to select the value to check distinct on
- `distinctKey` is removed as it is redundant
- `distinct` no longer supports a comparer function argument, as there is little to no use case for such an argument that could not be covered by the keySelector
- updates tests to remove tests that do not make sense and test new functionality

BREAKING CHANGE: `distinctKey` has been removed. Use `distinct`
BREAKING CHANGE: `distinct` operator has changed, first argument is an
optional `keySelector`. The custom `compare` function is no longer
supported.

resolves #2009

* perf(distinct): increase `distinct()` perf by improving deopts

- moves keySelector call to a different function with a try catch to improve V8 optimization
- distinct calls with no keySelector passed now take a fully optimized path, doubling speed again

related #2009

* docs(distinct): update distinct docs to fit new API

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