2.x: convert the Observable operators to return Single/Maybe

[akarnokd]

This PR updates many Observable operators to return Single, Maybe or Completable:

• count() -> Single
• elementAt() -> Maybe
• elementAt(T) -> Single
• first(T) -> Single
• firstElement() -> Maybe
• ignoreElements() -> Completable
• lastElement() -> Maybe
• reduce(BiFunction) -> Maybe
• reduce(Callable, BiFunction) -> Single
• reduceWith(U, BiFunction) -> Single
• single(T) -> Single
• singleElement() -> Maybe

and deletes Observable.toSingle, Observable.toMaybe and Observable.toCompletable.

In addition, Flowable.reduce(BiFunction) now returns Maybe<T>.

Related: #4321

[codecov-io]

Current coverage is 77.96% (diff: 75.78%)

Merging #4579 into 2.x will decrease coverage by 0.54%

@@                2.x      #4579   diff @@
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  Files           545        552     +7   
  Lines         35941      36164   +223   
  Methods           0          0          
  Messages          0          0          
  Branches       5548       5575    +27   
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- Hits          28216      28194    -22   
- Misses         5714       5950   +236   
- Partials       2011       2020     +9   

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[akarnokd]

Thanks @JakeWharton !