[swift-evolution] Standard ReactiveSteam definitions for Swift

[Georgios Moschovitis]

Copied from the corresponding Github issue, I would like to hear opinions from the broader community on this:

I am wondering if the correct 'translation' to Swift of:

```
onNext()
onError()
```

is really:

```
on(next:)
on(error:)
```

maybe something closer to Foundation's naming conventions would be:

```
didReceive(next:) or didReceive(value:)
didReceive(error:)
```

-g.