Improve tests-as-documentation
Improved mismatch descriptions for `hasItem`
Extract superclass HCDiagnosingMatcher for complex matchers
Improved mismatch descriptions for `onlyContains`
Improve test readability using NSNumber literals instead of strings, and array literals
Fix test support code for iOS Foundation
Remove unused imports
New matcher `everyItem` matches collections if every item satisfies a given matcher.
Make HCIsCollectionOnlyContaining a subclass of HCEvery
Clean up test