Functional Soft Heaps

[Chazelle-style Soft Heaps](http://en.wikipedia.org/wiki/Soft_heap) would be a great addition to this library. They are asymptotically optimal for a different class of problems than the Brodal/Okasaki heaps, and while a functional implementation cannot delete non-minimum nodes with the same O(1/epsilon) guarantee, you don't need that functionality for many of the algorithms that this structure is suited to. (soft minimal spanning trees, near sorting, exact selection with soft heaps, etc.)

Start by reading the existing Brodal/Okasaki heap implementations and the linked Chazelle-style Soft Heaps description. Define the functional operations and guarantees needed for soft minimal spanning trees, near sorting, and exact selection; done means the library includes a working functional soft heap suitable for those uses.

Written by the indexing model from the issue text.