Make `Head`, `Segbend`, `Bow` copyable

Having to move or borrow tem, especially `Head`, makes it really
difficult to appease the borrow checker.

src/bow.rs

@@ -5,6 +5,7 @@ use crate::graph::{
 };
 use crate::primitive::{Bend, Dot, Seg};
 
+#[derive(Debug, Clone, Copy)]
 pub struct Bow {
     seg1_dot1: DotIndex,
     seg1: SegIndex,

src/router.rs

@@ -27,6 +27,7 @@ struct Route {
     width: f64,
 }
 
+#[derive(Debug, Clone, Copy)]
 pub struct Head {
     pub dot: DotIndex,
     pub segbend: Option<Segbend>,

src/segbend.rs

@@ -5,6 +5,7 @@ use crate::{
     primitive::{Bend, Dot},
 };
 
+#[derive(Debug, Clone, Copy)]
 pub struct Segbend {
     pub seg: SegIndex,
     pub dot: DotIndex,