3.x: Disambiguate startWith(T|Iterable<T>)?

[akarnokd]

We were using postfixes to avoid inference problems around operator methods (such as fromIterable vs fromArray) but there is at least one operator currently, startWith, that may exhibit type ambiguity. A dedicated postfixed name should resolve the issue, such as:

1. startWith(T) + startWithIterable(Iterable<T>)
2. startWithItem(T) + startWith(Iterable<T>) + remove startWith(T)
3. startWithItem(T) + startWithIterable(Iterable<T>) + remove startWith(T).

As a preparation, we can deprecate startWith(T) within 2.2.x in case option 2 or 3 is chosen.

[vanniktech]

The same also happens with some onError* methods.

I'm more inclined towards option 3 but no strong opinion.

[davidmoten]

For 3.x I'm happy with 3 also.

[davidmoten]

Perhaps instead of startWithItem we use startWithJust to match our just usage?

[RomanWuattier]

That's just my two cents' worth I would opt for the option 3.

I think startWithJust could be confusing since other methods call just for example, range when the count value is one or fromArray when the length is one.

[akarnokd]

3.x is now accepting PRs if somebody wants to tackle this.

[akarnokd]

Closing via #6530.