Provide better indication of anonymous or logged in status

[therealbstern]

From #3:

We track the user that is associated with a ConnectionSignature, which is returned by the run method. That can be used as a key to store session information, although it can change, so is awkward on the application side. The get_user method returns the user logged in on that session, if any.

What @jolhoeft is thinking of is something to replace the run method, say get_connection, that returns an enum:

enum ConnectionType {
  Anonymous(ConnectionSignature),
  LoggedIn(ConnectionSignature, String),
}

We include the ConnectionSignature in the logged in case so the application can migrate information after the user logs in (or more likely, creates an account).

This can all be done on the application side now, but is a bit messy, and seems a common use case.

We'll also need:

fn upgrade_connection(c: ConnectionType, user: &str) -> ConnectionType

which consumes c, and returns a ConnectionType::LoggedIn if the user was not zero-length (otherwise it gives you back a ConnectionType::Anonymous again).

[therealbstern]

FWIW, the get_user method was designed to differentiate between logged-in users and anonymous users (as well as to identify the logged-in user specifically).

[therealbstern]

I sort of feel like this could be managed equally well with a "is this session authenticated" boolean that lives in the application, but if we want to own session variables, this is the right path to take.

Also I think we should own session variables, since this is "websession.rs", not "weblogin.rs".