Flexible routing

Another massive refactor to change the way routing works. Now any
column can route anywhere.

Also things are generally just much better and more modular via the
new struct split borrowing technique.

I didn't even try to split this into smaller commits for my sanity.

Signed-off-by: William Casarin <jb55@jb55.com>

src/view_state.rs

@@ -0,0 +1,13 @@
+use crate::login_manager::LoginState;
+
+/// Various state for views
+#[derive(Default)]
+pub struct ViewState {
+    pub login: LoginState,
+}
+
+impl ViewState {
+    pub fn login_mut(&mut self) -> &mut LoginState {
+        &mut self.login
+    }
+}