`pi` crashes for small u32 inputs due to subtraction overflow

[aschoettler]

I added these tests to my lib.rs:

#[test]
fn primepi_small_u32() {
    for i in (1u32..9) {
        i.pi();
    }
    assert_eq!(9u32.pi(), 4);
}
#[test]
fn primepi_ok_u32() {
    assert_eq!(9u32.pi(), 4);
}

The console output says

thread 'primepi_small_u32' panicked at 'attempt to subtract with overflow'

and points to the file sieve.rs:2:18

[JASory]

Temporarily fixed by treeing it down to 16-bit evaluation. Currently there are a fair amount of issues with handling very small integers as I haven't finished optimizing and treeing down the evaluations for very small integers. (Usually 0 and 1 are the failure points {due to definition of the factor function}, so watch out). I'll be working on this in the next week so most of those issues will be resolved.