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)

1 files changed, 2 insertions, 2 deletions

diff --git a/src/libextra/c_vec.rs b/src/libextra/c_vec.rs
index f72d3ee694f..e656360aa5a 100644
--- a/src/libextra/c_vec.rs
+++ b/src/libextra/c_vec.rs
@@ -122,7 +122,7 @@ pub unsafe fn c_vec_with_dtor<T>(base: *mut T, len: uint, dtor: @fn())
 pub fn get<T:Clone>(t: CVec<T>, ofs: uint) -> T {
     assert!(ofs < len(t));
     return unsafe {
-        (*ptr::mut_offset(t.base, ofs)).clone()
+        (*ptr::mut_offset(t.base, ofs as int)).clone()
     };
 }
 
@@ -133,7 +133,7 @@ pub fn get<T:Clone>(t: CVec<T>, ofs: uint) -> T {
  */
 pub fn set<T>(t: CVec<T>, ofs: uint, v: T) {
     assert!(ofs < len(t));
-    unsafe { *ptr::mut_offset(t.base, ofs) = v };
+    unsafe { *ptr::mut_offset(t.base, ofs as int) = v };
 }
 
 /*