README changes / code re-org for clarity

[wwoods]

A newcomer to this repo might find it difficult to locate the various parts of argo, or understand the point of the whole package. I've recently learned that argo is in fact a subprotocol of JSON RPC with reference implementations for the server and client, in Haskell and Python, respectively.

To help with this, the current argo folder could be renamed to argo-server-hs, and the python folder could be renamed to argo-client-py. This would be easy to spot, and would pair well with a PyPI module named argo_client. Generally, a Python module should share the same name as its source directly, though in this case the -py would be helpful since the argo repo is really a repository of reference implementations.

The README also needs an update to explicitly state that argo is a subprotocol for JSON-RPC, and that the repository contains reference implementations of the server in Haskell and the client in Python.

This is separate from, but complementary to, excising all of the SAW/Cryptol-specific code from this repository.