Singular() method to force operation to be not partitioned

Sometimes it is really needed to have a stateful map (or any other) operation, in this case, it can't be partitioned (automatically), and needed to be marked as `SinglePartitionBean`. Right now you may use `flatMap()` returning singleton list, but that way is hacky and non-consistent.

Proposed signature:
```kotlin
stream.map { }.singular()
```