api: impl Clone for ByteRecordIter and StringRecordIter

Cloneable iterators are useful when writing code that is generic over
iterators that needs to be able to iterate twice.

PR #204

src/byte_record.rs

@@ -761,6 +761,7 @@ impl<T: AsRef<[u8]>> Extend<T> for ByteRecord {
 ///
 /// The `'r` lifetime variable refers to the lifetime of the `ByteRecord` that
 /// is being iterated over.
+#[derive(Clone)]
 pub struct ByteRecordIter<'r> {
  /// The record we are iterating over.
  r: &'r ByteRecord,

src/string_record.rs

@@ -699,6 +699,7 @@ impl<'a> IntoIterator for &'a StringRecord {
 ///
 /// The `'r` lifetime variable refers to the lifetime of the `StringRecord`
 /// that is being iterated over.
+#[derive(Clone)]
 pub struct StringRecordIter<'r>(ByteRecordIter<'r>);
 
 impl<'r> Iterator for StringRecordIter<'r> {