Increase Chain test coverage.

Chain coverage:
	Lines: 76% -> 100%
	Functions: 57.14% -> 93.33%
Total coverage:
	Lines: 82.32% -> 83.9%
	Functions: 46.33% -> 49.41%

tests/test_chain.rs

@@ -1,4 +1,4 @@
-use anyhow::{anyhow, Error};
+use anyhow::{anyhow, Chain, Error};
 
 fn error() -> Error {
     anyhow!(0).context(1).context(2).context(3)
@@ -33,13 +33,36 @@ fn test_len() {
     let e = error();
     let mut chain = e.chain();
     assert_eq!(4, chain.len());
+    assert_eq!((4, Some(4)), chain.size_hint());
     assert_eq!("3", chain.next().unwrap().to_string());
     assert_eq!(3, chain.len());
+    assert_eq!((3, Some(3)), chain.size_hint());
     assert_eq!("0", chain.next_back().unwrap().to_string());
     assert_eq!(2, chain.len());
+    assert_eq!((2, Some(2)), chain.size_hint());
     assert_eq!("2", chain.next().unwrap().to_string());
     assert_eq!(1, chain.len());
+    assert_eq!((1, Some(1)), chain.size_hint());
     assert_eq!("1", chain.next_back().unwrap().to_string());
     assert_eq!(0, chain.len());
+    assert_eq!((0, Some(0)), chain.size_hint());
     assert!(chain.next().is_none());
 }
+
+#[test]
+fn test_default() {
+    let mut c: Chain = Default::default();
+    assert!(c.next().is_none());
+}
+
+#[test]
+fn test_clone() {
+    let e = error();
+    let mut chain = e.chain().clone();
+    assert_eq!("3", chain.next().unwrap().to_string());
+    assert_eq!("2", chain.next().unwrap().to_string());
+    assert_eq!("1", chain.next().unwrap().to_string());
+    assert_eq!("0", chain.next().unwrap().to_string());
+    assert!(chain.next().is_none());
+    assert!(chain.next_back().is_none());
+}