Make vec::zip not require T:copy

[bblum]

Currently we have:

pure fn zip<T: copy, U: copy>(v: &[const T], u: &[const U]) -> ~[(T, U)]

Produces an "instantiating copy type parameter" warning when building stdtest.

We could instead have:

pure fn zip_ref<T: copy, U: copy>(v: &[const T], u: &[const U]) -> ~[(T, U)]
pure fn zip<T, U>(+v: ~[const T], +u: ~[const U]) -> ~[(T, U)]

[catamorphism]

I'm a bit puzzled about the second proposed function signature (for zip), as this doesn't allow the copy of v[i] and u[i] in vec::zip -- was your intention for zip to return a vector of pairs of references?

[bblum]

My thought was if zip owned its input, it would consume them when it put the elements in the new vector. something like:

pure fn zip<T, U>(+v: ~[const T], +u: ~[const U]) -> ~[(T, U)] {
    let i = len(v);
    assert i == len(u);
    let w = ~[];
    while i > 0 {
        push(w, (pop(v),pop(u)));
    }
    reverse(w);
    w
}

[bblum]

I did unzip too. I renamed the existing ones to zip_slice and unzip_slice.