Fast conversion to/from ASCII/one-byte strings?

[dead-claudia]

It's a near universal optimization within JS implementations to split strings into two representations: a one-byte and a two-byte. Even the size-optimized runtime XS does it. It brings a massive speed boost and almost halves string memory usage in practice. So, I'd like to see fromCharByteArray and intoCharByteArray that return character codes modulo 256.