Perf hack

I don't know why, but this weird trick speeds up `unsafeUpdate` by almost 50%. Benchmarks drop from 7.5 microseconds to 4.1 microseconds on my machine. This is such a fucking weird speedup that I want to know if it works for other people also. 

I don't know why it works, nor do I know why it fails for `unsafeValueAt`. 

I may have to force the JVM to emit some assembler so I can figure it out.