fix round2_32

[Duncaen]

There was a missing import in round2_32.go.

I split the tests into per int size files too, since unsafe.Sizeof(int(0)) == 4 runtime does not allow to pass 64 bit constants to 32 bit functions at compile time. Not sure if there is a better solution.

[Duncaen]

round2_64 is missing x |= x >> 32 not sure if this is intentional because of the 100MB cut-off.

[erikdubbelboer]

Thanks!