0) Garbage collection. Under some extreme circumstances
   in which the number of lost characters is
   intolerably high, ne should allocate a new block of
   memory and move contiguously the text into it.

1) Fast large copying. When one pastes a *very large*
   clip, we should create a pool entry just for that
   copy and copy contiguously in it, instead of
   allocating line by line.

2) Check that update_partial_line() is always given the best start column.

3) Rewrite SHIFT to use direct buffer access rather than cursor movements.
