Make generic_ functions taking &impl IntoIter more generic

[oberien]

The original functions could not take references to slices and were somewhat limiting in their input. This change makes them more generic by allowing more types to be passed into them.

While this is technically a breaking change, because code which explicitly states the generics won't compile anymore, all other code should be unaffected. Thus, I expect pretty much no code to break in the wild. A minor version increase might be a good idea anyways.

[maxbachmann]

This certainly makes sense. Internally we already run into this in the string versions. They use StringWrapper only to workaround this. I would consider this a breaking change though.