Avoid copying childNodes to insert characters.

- Big performance bottleneck.
More quickly append long characters to NSMutableString.

- Was a performance bottleneck.
Unroll or memoize arrays in "in body" insertion mode.

- Spent too much time (de)allocating these arrays. Make them once or not at all.
Remove pointless indirection.

- Wasn't particularly high on the list of time-wasters but it's just too
  offensive.