Implement FromStr for ArrayString

This is very useful for generic code that may want to parse arbitrary
input string into arbitrary other types.

Limitations of the FromStr trait don't allow us to keep the original string slice
inside the CapacityError, unfortunately.

tests/tests.rs

@@ -487,6 +487,14 @@ fn test_string_from() {
     assert_eq!(u.len(), text.len());
 }
 
+#[test]
+fn test_string_parse_from_str() {
+    let text = "hello world";
+    let u: ArrayString<[_; 11]> = text.parse().unwrap();
+    assert_eq!(&u, text);
+    assert_eq!(u.len(), text.len());
+}
+
 #[test]
 fn test_string_from_bytes() {
     let text = "hello world";