Add Cow::make_mut

[michaelsproul]

It is useful to be able to upgrade a Cow pointer into its mutable variant without consuming it, which is what Cow::to_mut does. This PR adds a new method Cow::make_mut which makes the Cow pointer mutable, and returns it. The returned reference has a shorter lifetime than the one returned by to_mut because it borrows from &mut self rather than self.

The to_mut method has been renamed into_mut as we needed to change its type signature anyway to introduce a Result type. The new make_mut also returns a Result.