Async collection mapping

[iamandylamb]

The Mapper includes two async collection mapping methods. MapAsync and MapParallel.
MapAsync maps each item sequentially, but the whole collection asynchronously.
MapParallel maps each item asynchronously, wrapping all mapping tasks in a WhenAll task to give the same signature as MapAsync but using a different approach.

Whether this is useful or not, I'm not sure. It may just be adding complication for not much benefit.
Need to think about:

• Naming - should it be MapParallelAsync? Or something else?
• Documentation - If it's staying, it'll need some really clear documentation.

[iamandylamb]

Some documentation added to README.md