Make Map/Set subclassable; support `iterable` argument to constructor.

Use the same technique added for `Promise` to kludge in support for
ES6-style construction and subclassing, factoring out some helper
methods in the process.  Add support for the `iterable` argument to
Map and Set.
Merge pull request #218 from cscott/subclassable-map

Make Map/Set subclassable; support `iterable` argument to constructor.