Open Radar

 
Summary:
Swift.String, Swift.Array, and Swift.Set should all conform to a single protocol supporting the following:
* Enumerating the elements in the collection
* Removing elements found by enumerating
* Adding elements (appending in an ordered collection)
* Testing for the presence of an element

Currently, String and Array conform to ExtensibleCollectionType, but Set does not.

Steps to Reproduce:
1. Download and add my CollectionDictionary type to a project: <https://gist.github.com/brentdax/f8a856ccd9a9fa5ab86f>.
2. Extend it to support a Value type of Swift.Set in addition to Swift.Array and Swift.String.

Expected Results:
Doing so is possible because Swift.Set conforms to either ExtensibleCollectionType or a similar protocol with less stringent “informal” requirements.

Actual Results:
Doing so is not possible because Array, String, and Set do not support any common protocols which allow mutation.

Notes:
There are certainly semantic and time complexity differences between Swift.Set’s insert() and the other types’ append() methods, but in many use cases, these differences don’t matter. We also need a way to handle these sorts of situations.