Consider refactoring Pausable API

Currently, every fungible faucet account installs the `is_paused` storage slot via `Pausable` and the `pause / unpause` admin procedures `PausableManager` as two separate `.with_component()` calls, which creates an issue forgetting the manager: The slot exists but pause is unreachable, or the procedures exist but the slot is missing and they panic at runtime.

The two components are practically inseparable. We can follow the same convention already used by `AccessControl::Ownable2Step` and `AccessControl::Rbac`, which expand into multiple components with a single `.with_components()` call, by having:

`Pausable` implements `IntoIterator`.