feat(source): Remove bound T: Clone from Source implementation fo…

…r `Cow`. (#42)

This change removes the bound `T: Clone` in the implementation of
`Source` for `Cow<'_, T>`. This expands the implementation of `Source`
to include types like `Cow<'_, str>`. Importantly, `Cow` always requires
the bound `T: ToOwned` and uses `ToOwned` to implement `Clone` rather
than forwarding to `T::clone`.

src/source_impls.rs

@@ -1,7 +1,11 @@
 /*!
 Default trait implementations for [Source].
 */
-use std::{borrow::Cow, sync::Arc};
+use std::{
+    borrow::{Cow, ToOwned},
+    fmt::Debug,
+    sync::Arc,
+};
 
 use crate::{MietteError, MietteSpanContents, Source, SourceSpan, SpanContents};
 
@@ -84,7 +88,12 @@ impl<T: Source> Source for Arc<T> {
     }
 }
 
-impl<T: Source + Clone> Source for Cow<'_, T> {
+impl<T: ?Sized + Source + ToOwned> Source for Cow<'_, T>
+where
+    // The minimal bounds are used here. `T::Owned` need not be `Source`, because `&T` can always
+    // be obtained from `Cow<'_, T>`.
+    T::Owned: Debug + Send + Sync,
+{
     fn read_span<'a>(
         &'a self,
         span: &SourceSpan,