Implement FastBytes, and use it for MachStr
This is a first step on the way to refactoring the FastString type. FastBytes currently has no unique, mainly because there isn't currently a nice way to produce them in Binary. Also, we don't currently do the "Dictionary" thing with FastBytes in Binary. I'm not sure whether this is important. We can change both decisions later, but in the meantime this gets the refactoring underway.
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