MergeWith, ConcatWith, AmbWith

Single-arity instance versions of merge, concat and amb.
Since we can't use the `merge`/`concat`/`amb` names on both static and instance methods, these have the `With` suffix, which actually reads quite well.

```java
o.mergeWith(o2).subscribe(ts);
o.concatWith(o2).subscribe(ts);
o.ambWith(o2).subscribe(ts);
```

If we feel the need for more arities fine, but I wanted to start simple.

This originated from discussions with @headinthebox and over a year of wanting instance methods for `merge`.