SIMD Within a Register (SWAR) on Strings in Go 1.20

To introduce SWAR (SIMD within-a-register) techniques in go, we’ll implement the simplest thing I can think of: checking if a string is ASCII. SWAR is the technique of operating over multiple datapoints within one register of the cpu. In our case one 64-bit variable will contain 8 bytes of our string so we can process 8 bytes at a time. We’ll do this in go without any C functions or assembly.