Make Branch constructor consistent with other composite runnables

[randomir]

While other composing runnables take children runnables as positional arguments (children-accepting: Branches, Parallel, Race; child-accepting: Loop, Map, Unwind), Branch takes its children in a single argument, components.

Although there are benefits of this approach (lazy composition/iteration, explicitness), and Branch constructor is rarely used explicitly (usually implicitly with | operator), the inconsistency can be confusing for users (see #207).