String#insert

    String#insert with index, other
      - inserts other before the character at the given index
      - modifies self in place
      - inserts after the given character on an negative count
      - raises an IndexError if the index is beyond string
      - converts index to an integer using to_int
      - converts other to a string using to_str
      - raises a TypeError if other can't be converted to string

`String#insert` is necessary to run core/process/kill_spec.rb.