Scipy 1.0 Roadmap

Note: to be discussed on the list, don't merge.
With `std::map` as implemented by GNU, I could store the entire Wikipedia link graph in a few GB of RAM. Memory overhead can maybe be suppressed by using a custom allocator.

I've also experimented with `boost::unordered_map`, which gave much slower iteration, and [Google sparsehash](https://code.google.com/p/sparsehash/), which didn't buy me much compared to `std::map` (it's easily backported into SciPy, but deleting element is problematic with that lib).

Re @perimosocordiae that format with a "fancy data structure" is what I had in mind for DOK. It would be backward incompatible with current COO, though, and a major selling point of COO is its cheap transpose which will be hard to preserve.