insert in string-snip%

Inserts text into the snip. The system can insert text into a text snip without calling this method.

• (send a-string-snip insert s len pos) -> void
     s : string
     len : exact non-negative integer
     pos = 0 : exact non-negative integer

  Inserts s (with length len) into the snip at relative position pos within the snip.