Various changes.

BREAKING CHANGE:

* **iam**: rename `ImportedResourcePrincipal` to `UnknownPrincipal`.
* **iam**: `managedPolicyArns` renamed to `managedPolicies`, takes
  return value from `ManagedPolicy.fromAwsManagedPolicyName()`.

Remove `postProcess` from Token interface, move opting into it into
`IResolvable.resolve()`, so it can be hidden from the `PolicyDocument`
public interface.

Make addStatements() variadic.

PolicyStatement is no longer IResolvable (doesn't need to be)

Remove AwsPrincipal, which was always an alias for ArnPrincipal
which is much clearer.

Introduce `PolicyStatement.fromAttributes({ ... })` as an
alternative to the fluent interface chaining of old `PolicyStatement`.