XRef and XRefMut

When you want to return some data that might be either owned or borrowed, it is common to return a Cow<T>.

However, Cow<T> has the following limitations:

• T has to be ToOwned, and
• if the data you want to borrow is behind a RefCell, you can't.

XRef<T> is like Cow<T>, except addresses the above limitations with the following differences:

• T does not have to be ToOwned, (this means that now the type contained in Borrowed and Owned have to contain the exact matching types), and
• XRef<T> has a third variant Ref which holds a std::cell::Ref<T>, for cases when the value is borrowed from a RefCell.

XRefMut<T> is like XRef<T> except that

• all references are borrowed mutably (i.e. XRef::Borrowed holds a &mut T instead of &T, XRef::Ref holds a RefMut<T> instead of a Ref<T>),
• XRefMut<T> implements DerefMut<T>, to allow borrowing data mutably without cloning.