Implement extremely basic combat

There's a gormlak, you can kill it.
That's it.

src/entities/entity.rs

@@ -0,0 +1,87 @@
+use crate::display::Draw;
+use crate::entities::EntityID;
+use crate::types::{Positioned, PositionedMut};
+use downcast_rs::Downcast;
+use std::fmt::Debug;
+use std::io::{self, Write};
+
+pub trait Identified<ID>: Debug {
+    fn opt_id(&self) -> Option<ID>;
+    fn set_id(&mut self, id: ID);
+
+    fn id(&self) -> ID {
+        self.opt_id()
+            .expect(format!("Entity ({:?}) is not in the game", self).as_str())
+    }
+}
+
+impl<'a, A, ID> Identified<ID> for &'a mut A
+where
+    A: Identified<ID>,
+{
+    fn opt_id(&self) -> Option<ID> {
+        (**self).opt_id()
+    }
+    fn set_id(&mut self, id: ID) {
+        (**self).set_id(id);
+    }
+}
+
+impl<ID, A: Identified<ID>> Identified<ID> for Box<A> {
+    fn opt_id(&self) -> Option<ID> {
+        (**self).opt_id()
+    }
+    fn set_id(&mut self, id: ID) {
+        (**self).set_id(id);
+    }
+}
+
+pub trait Entity:
+    Positioned + PositionedMut + Identified<EntityID> + Draw + Downcast
+{
+}
+
+impl Identified<EntityID> for Box<dyn Entity> {
+    fn opt_id(&self) -> Option<EntityID> {
+        (**self).opt_id()
+    }
+    fn set_id(&mut self, id: EntityID) {
+        (**self).set_id(id);
+    }
+}
+
+#[macro_export]
+macro_rules! identified {
+    ($name: ident, $typ: ident) => {
+        identified!($name, $typ, id);
+    };
+    ($name: ident, $typ: ident, $attr: ident) => {
+        impl crate::entities::entity::Identified<$typ> for $name {
+            fn opt_id(&self) -> Option<$typ> {
+                self.$attr
+            }
+
+            fn set_id(&mut self, id: $typ) {
+                self.$attr = Some(id)
+            }
+        }
+    };
+}
+
+#[macro_export]
+macro_rules! entity {
+    ($name: ident) => {
+        positioned!($name);
+        positioned_mut!($name);
+        identified!($name, EntityID);
+        impl crate::entities::entity::Entity for $name {}
+    };
+}
+
+impl_downcast!(Entity);
+
+impl Draw for Box<dyn Entity> {
+    fn do_draw(&self, out: &mut Write) -> io::Result<()> {
+        (**self).do_draw(out)
+    }
+}