Implement lifetime-free ref and mut casts for unsized type

[zheland]

The TryCastMutLifetimeFree and TryCastRefLifetimeFree trait implementations have default Sized bound on the T and U generics, which prevents unsized types not defined as static from casiting.

This PR adds ?Sized bound to T and U generics in TryCastMutLifetimeFree and TryCastRefLifetimeFree trait implementation.

For example this makes it possible to specialize slices without restricting the source slice item type to be static:

fn func<T>(slice: &[T]) {
    if let Ok(byte_slice) = cast!(slice, &[u8]) {
        // specialized impl...
    } else {
        // default impl...
    }
}

[sagebind]

Thanks for the idea and working on this! If that's alright, I am going to release #25 as a more urgent patch release, and then review this PR a bit later to include in a subsequent minor version feature release.

[zheland]

Rebased to upstream/master.