Switch from slice to slice-of-refs for spend_spendable_outputs

Switch from slice to slice-of-refs for spend_spendable_outputs

Sadly, there's just not really a practical way to map a slice of
objects in our current bindings infrastructure - either we take
ownership of the underlying objects and move them into a Vec, or we
need to leave the original objects in place and have a list of
pointers to the Rust objects. Thus, the only practical mapping is
to create a slice of references using the pointers we have.