Remove `Into<&[u8]>` bound from `IntoByteSlice`?

[joshlf]

The Into<&[u8]> bound on IntoByteSlice means that we can't implement IntoByteSlice for &mut [u8]. If instead we used a method directly on IntoByteSlice, we could remove this restriction.

This restriction means, for example, that Ref::into_ref isn't available for Ref<&mut [u8], _>.