mutable strings

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it would be nice if there was a mutable string that can handle the full range of values that regular immutable strings can.  for now, i convert them to a list, work on them as needed, and finally join it all back to a regular immutable string (if i need the result as a string).  mutable strings would probably be no more efficient than a list,  but, having them could make for cleaner more readable code by not having the conversions coded in there.