Support mapToMany for Single

### Name and description

Support mapMany for Single

### Motivation for inclusion

It is very annoying to convert to Observable to be able to use mapMany and then to convert it to single.

### Example of use

```
mySingle.mapMany(...)
```

In place of
```
mySingle.asObservable()
               .mapMany(...)
               .asSingle()
```
