Opaque Public Context (#576)

Make the execution context presented to users an opaque handle, moving all implementation of the gpu, thread and distributed contexts into the back end.

* move `execution_context` and `distributed_context` definitions to the back end
* create `execution_context` handle called `context` in the public API
* provide `make_context` helper functions that build different context configurations (default, user-specified local resources, with MPI)
* update documentation for all parts of the public API that touch contexts
* move `distributed_context` docs to the developer documentation (from the public API docs)