Implement cast from lifetime-free to unbound types

[zheland]

The TryCast{Mut,Ref,Owned}LifetimeFree traits are implemented for LifetimeFree target types, but not implemented for LifetimeFree source types.

This PR adds TryCast{Mut,Ref,Owned}LifetimeFreeBack traits with source types bounded by LifetimeFree trait.

This makes it possible to upcast types and make specialized builders/decoders/constructors like this:

fn decode_smth<T, const N: usize>(buffer: &[u8]) -> [T; N] {
    fn specialized_impl<const N: usize>(buffer: &[u8]) -> [u8; N] { todo!() }
    fn default_impl<T, const N: usize>(buffer: &[u8]) -> [T; N] { todo!() }

    if cast!(0_u8, T).is_ok() {
        let byte_array: [u8; N] = specialized_impl(buffer);
        cast!(byte_array, [T; N]).expect("unexpected cast failure")
    } else {
        default_impl(buffer)
    }
}

This PR is based on (includes) another PR #23 and might require rebase before merging if needed. This PR includes additional tests for both PRs. You can use last commit (7b75e1e if not changed) for better diffs related to this PR changes only.

Common implementations for *LifetimeFree::try_cast and *LifetimeFreeBack::try_cast methods are extracted into separate private functions to reduce boilerplate code.

New autoderef ordering: TryCastMutLifetimeFree, TryCastMutLifetimeFreeBack (new), TryCastRefLifetimeFree, TryCastRefLifetimeFreeBack (new), TryCastOwnedLifetimeFree, TryCastOwnedLifetimeFreeBack (new), TryCastSliceMut, TryCastSliceRef, TryCastMut, TryCastRef, TryCastOwned