Overflow issue

[Phlosioneer]

So I figured out the issues I was having yesterday. I checked my sha1 against yours, and it matches perfectly with yours. Investigating further, I figured out that there was an integer overflow occurring in the addi instruction. I don't know why your rust didn't catch it, while mine did, but the code, pif, and rom are exactly the same.

In any case, I was forced to implement some small amount of overflow handling in #51. It might help anyone else out there who has the same issue. I'm not sure why rust was overflowing - I think it was because it was trying to add two u64's that represented negative numbers. Not sure if that means i64 would have been more appropriate, or i32, or whatever. I just implemented it bit-wise, checking for overflow manually.

Now that I think about it, I think my implementation breaks the 64bit-mode. I don't know. Thoughts?

[yupferris]

I've left a comment with my thoughts on the PR.