Remove functions like vec::iter_between

[erickt]

Now that we have slices, we can replace functions like vec::iter_between(v, 5u, 10u) { |x| ...} with vec::iter(vec::view(v, 5u, 10u)) { |x| ... }.

[eholk]

Would it maybe make more sense to reimplement vec::iter_between in terms of iter and view? That way we don't break any existing code (although I'm not sure how much of that there is).

[catamorphism]

vec::iter_between seems to be no longer. Closing.