Kernel specs REST API

The [REST API for kernel specs](https://github.com/ipython/ipython/wiki/IPEP-25:-Registry-of-installed-kernels#rest-api) is described in IPEP 25.
Not much in it, to my mind. I might think `as_*` returned a view of the object that updated in sync with the original, rather than a static copy of its data. But I certainly wouldn't rely on that without checking it.