initial version of DictionaryProtocol

[JaapWijnen]

This is a first attempt at implementing a DictionaryProtocol protocol as discussed in #293
This is not complete yet and needs discussion in order to improve!
For now it conforms stdlib Dictionary and OrderedDictionary to the newly introduced DictionaryProtocol.
A basic first definition of a MutableDictionaryProtocol has been created but no conformances added yet. (Want to get the base protocol right first and perhaps identify methods that we would want a mutable version to have)
Hoping to get some feedback on the current implementation and will add conformances to the other Dictionary objects next.

Would we also like to introduce a SetProtocol? Should we consider this first since Dictionaries often depend on some type of Set of keys? Or do we keep the keys collection more generic than constraining it to a Set kind of type?

Not all checks are done since this is still in development :) Would love to hear people's thoughts!

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[JaapWijnen]

Ah forget about the SetProtocol remark. Forgot that the regular Dictionary's Keys is not strictly a Set.

[JaapWijnen]

@lorentey Would you have a bit of direction for me? I'd love to get this to the finish line but could really use some guidance from you. See also my comments in #293