Contain nested and subviews

This adds support for a generic nested() modifier to be used with the contain matcher. It also adds specializations to the contain matcher for UIView and CALayer objects, allowing for searching within their hierarchies as well. Note that in contrast to how the stories(#68773034, #68359004) are written, this is not adding any additional components to the DSL but rather allows the view to be treated as a container directly.

(See also @joemasilotti's [pull request](https://github.com/pivotal/cedar/pull/199) for context and contrast)