api: implement Clone for SubCaptureMatches

This is useful in a generic context when you want to iterate over
something twice. There's not much reason to not do it at least.

Closes #689

src/re_bytes.rs

@@ -1056,6 +1056,7 @@ impl<'t, 'i> Index<&'i str> for Captures<'t> {
 ///
 /// The lifetime `'c` corresponds to the lifetime of the `Captures` value, and
 /// the lifetime `'t` corresponds to the originally matched text.
+#[derive(Clone)]
 pub struct SubCaptureMatches<'c, 't: 'c> {
     caps: &'c Captures<'t>,
     it: SubCapturesPosIter<'c>,

src/re_trait.rs

@@ -51,6 +51,7 @@ impl Locations {
 /// Positions are byte indices in terms of the original string matched.
 ///
 /// `'c` is the lifetime of the captures.
+#[derive(Clone)]
 pub struct SubCapturesPosIter<'c> {
     idx: usize,
     locs: &'c Locations,

src/re_unicode.rs

@@ -1058,6 +1058,7 @@ impl<'t, 'i> Index<&'i str> for Captures<'t> {
 ///
 /// The lifetime `'c` corresponds to the lifetime of the `Captures` value, and
 /// the lifetime `'t` corresponds to the originally matched text.
+#[derive(Clone)]
 pub struct SubCaptureMatches<'c, 't: 'c> {
     caps: &'c Captures<'t>,
     it: SubCapturesPosIter<'c>,