Add as_borrowed function

[Licenser]

The function will consume the Cow and return the borrowed data with the associated lifetime. It panics if the data wasn't borrowed.

closes #19

[CAD97]

This operation would probably be best called unwrap_borrowed as currently written.

If given a choice, I'd expect:

    fn as_borrowed(&self) -> Option<&'a T> {
        match self {
            Cow::Owned(_) => None,
            Cow::Borrowed(borrowed) => Some(borrowed),
        }
    }

    fn as_owned(&self) -> Option<&T::Owned> {
        match self {
            Cow::Owned(owned) => Some(owned),
            Cow::Borrowed(_) => None,
        }
    }

    fn as_owned_mut(&mut self) -> Option<&mut T::Owned> {
        match self {
            Cow::Owned(owned) => Some(owned),
            Cow::Borrowed(_) => None,
        }
    }

[Licenser]

that's a much better name! I'll update it to unwrap_borrowed :)

[maciejhirsz]

Thanks for the PR! Released as 0.4.1.