implement pointer arithmetic with GEP

Closes #8118, #7136

~~~rust
extern mod extra;

use std::vec;
use std::ptr;

fn bench_from_elem(b: &mut extra::test::BenchHarness) {
    do b.iter {
        let v: ~[u8] = vec::from_elem(1024, 0u8);
    }
}

fn bench_set_memory(b: &mut extra::test::BenchHarness) {
    do b.iter {
        let mut v: ~[u8] = vec::with_capacity(1024);
        unsafe {
            let vp = vec::raw::to_mut_ptr(v);
            ptr::set_memory(vp, 0, 1024);
            vec::raw::set_len(&mut v, 1024);
        }
    }
}

fn bench_vec_repeat(b: &mut extra::test::BenchHarness) {
    do b.iter {
        let v: ~[u8] = ~[0u8, ..1024];
    }
}
~~~

Before:

test bench_from_elem ... bench: 415 ns/iter (+/- 17)
test bench_set_memory ... bench: 85 ns/iter (+/- 4)
test bench_vec_repeat ... bench: 83 ns/iter (+/- 3)

After:

test bench_from_elem ... bench: 84 ns/iter (+/- 2)
test bench_set_memory ... bench: 84 ns/iter (+/- 5)
test bench_vec_repeat ... bench: 84 ns/iter (+/- 3)