Bounds checking code can overflow

[Thiez]

When using very large indices, the bounds checking code fails. Example:

let x = [0i32, ..16];
assert_eq(x[1u << 63],0);
assert_eq(&x[0], &x[1u << 63]);

Both asserts pass on a 64 bit system.

[thestinger]

It should compare/store element length rather than byte length.