Adding and moving a lot of components.

This is the main thrust of the library. I know it is a lot of changes,
but I was running out of time and had to hammer them all in.

bard/src/togglable.rs

@@ -0,0 +1,37 @@
+use std::collections::HashSet;
+
+
+
+/// Trait to toggle the presence of a value (like a tag) in a set.
+pub trait Togglable
+{
+   type Item: ?Sized;
+
+   fn toggle(&mut self, item: &Self::Item);
+   fn is_toggled(&self, item: &Self::Item) -> bool;
+}
+
+
+
+impl Togglable for HashSet<String>
+{
+   type Item = str;
+
+
+   fn toggle(&mut self, item: &str)
+   {
+      if self.contains(item)
+      {
+         self.remove(item);
+      }
+      else
+      {
+         self.insert(item.to_owned());
+      }
+   }
+
+   fn is_toggled(&self, item: &str) -> bool
+   {
+      self.contains(item)
+   }
+}