Rust: create an "overview" spec

[brson]

Every project contains a "spec" beforehand, of various levels of detail. A full standalone spec representing the final project would help in a few ways, but primarily it would help me maintain continuity between the courses.

As an example, I didn't realize it until recently that the existence of KvsClient and KvsServer are specified, but nothing about their APIs. This is purposeful: it makes it possible to talk about them, but leaves the implementation strategy, which would be reflected in the API, entirely up to the student. This is a "rule" about the project that should be captured somewhere, as I had been inconsistent about it between projects, making assumptions about methods on them.