Don't collect other agents' position for worldview

This is a huge performance improvement (drops ~90% of execution time)
with the current SimpleAgent implementation, because a SimpleAgent
that is not It will not even look at other agents' positions.

src/agent.rs

@@ -26,12 +26,12 @@ impl From<(Distance, Distance)> for Direction {
 
 pub type AgentId = usize;
 
-pub struct WorldView {
+pub struct WorldView<'o> {
   pub self_id: AgentId,
   pub tagged: AgentId,
   pub tagged_by: Option<AgentId>,
   pub bounds_distance: (Distance, Distance, Distance, Distance),
-  pub other_agents: Vec<(Direction, AgentId)>,
+  pub other_agents: &'o mut dyn Iterator<Item = (Direction, AgentId)>,
 }
 
 #[derive(Debug)]

src/world.rs

@@ -58,7 +58,7 @@ impl ActualWorld {
             tagged_by: self.state.tagged_by,
             tagged: self.state.tagged,
             bounds_distance: (pos.y, self.size.x - pos.x, self.size.y - pos.y, pos.x),
-            other_agents: self
+            other_agents: &mut self
               .state
               .agent_positions
               .iter()
@@ -71,8 +71,7 @@ impl ActualWorld {
                   },
                   *id,
                 )
-              })
-              .collect(),
+              }),
           }),
         )
       })