Fix u64 and i64 in Safari 14

[Firaenix]

This PR removes the use of BigUint64Array and BigInt64Array as Safari 14 does not support them. I have also run all the u64 and i64 tests and they work as expected.

[alexcrichton]

Thanks for the PR here on this, but I think that much of this can be simplified. It looks like this is having numbers travel through the u32 array but I don't think that's necessary if the numbers are already manually split. Additionally I don't think the u64 conversion "shims" should really be necessary any more.

[Firaenix]

Sorry for the late response, life gets in the way sometimes.

I think you're right about the U32 Array, I can remove those. In regards to the u64 conversion shims, I can remove those too, just need to do some testing.

Thanks for the feedback.

[Liamolucko]

This got fixed in #3037.