Replace `<~` operator with `bind(with:)`

[reactivecocoa]

Replace `<~` operator with `bind(with:)`

[kaybutter]

The operator does express a direction though. The arrow tells you what goes where. The word "bind" doesn't really express a direction, even though the method probably won't setup a two-way binding. 🤔

[liscio]

Yeah, originally when I started out I was weary of the <~ operator because it seemed strange. However, as @kaybutter raised it's difficult to express directionality without the operator—especially because bindings are (and will be for a long time) unidirectional.

We expect that new users will at least have some initial familiarity with the API (via the README, etc.) by the time they've started working with the framework so I'm not concerned that the operator will be completely foreign.