Solving the mutating slice CoW problem

[dabrahams (Dave Abrahams)]

Yes, maintaining only a single reference doesn't keep this property:

@dabrahams

This approach opens the door to just the kind of mutation we want to allow, without disturbing the logic that makes CoW correct everywhere else.

If the slice is grown by the mutation, for example, it will release its buffer, and with only a single reference that's going to deallocate it and anything it was sliced out of.