api: impl Clone for Utf8Error

This matches what std::str::Utf8Error has, and it seems like a common
sense impl.

We do not add a 'Copy' impl however, since that seems like a backcompat
hazard. Although, it does seem unlikely that this error type will ever
*not* be 'Copy', it's not clear to me that 'Copy' makes semantic sense
for this type.

Closes #113

src/utf8.rs

@@ -402,7 +402,7 @@ impl<'a> ::core::iter::FusedIterator for Utf8Chunks<'a> {}
 /// assert_eq!(err.valid_up_to(), 6);
 /// assert_eq!(err.error_len(), Some(1));
 /// ```
-#[derive(Debug, Eq, PartialEq)]
+#[derive(Clone, Debug, Eq, PartialEq)]
 pub struct Utf8Error {
     valid_up_to: usize,
     error_len: Option<usize>,