Mutate f-strings and concatenated strings #439
Description
Our operator_string currently only mutates cst.SimpleString, but not cst.FormattedString and cst.ConcatenatedString.
I think we should also mutate those, similar to the simple string. I would only mutate one (the first non-empty?) part of the string parts. The other elements can be kept the same, because I think mutating one location per string is enough to check if it is covered by a test, and leaving the others out we can reduce the number of mutants.