Implement a global map of entities

Implement a global map of entities, which allows referencing by either
position or ID and updating the positions of existent entities, and put
the character in there.

src/entities/mod.rs

@@ -1,2 +1,16 @@
 pub mod character;
+use crate::display::Draw;
+use crate::types::{Positioned, PositionedMut};
 pub use character::Character;
+use downcast_rs::Downcast;
+use std::io::{self, Write};
+
+pub trait Entity: Positioned + PositionedMut + Draw + Downcast {}
+
+impl_downcast!(Entity);
+
+impl Draw for Box<dyn Entity> {
+    fn do_draw(&self, out: &mut Write) -> io::Result<()> {
+        (**self).do_draw(out)
+    }
+}