Add String#insert
Assert that index in String.insert is non-negative
String.insert: fix the error message and indentation
Another String#insert example passes
Move the logic of String#insert out of the strategy
Merge 625ece1d8a0f8fecac43c1cf61d4866fcf83b26b into a9d8a4fc1494b11e6760df8b770e0eae51db1b13
some cleanup