New code to grow and shrink buffers guarantees that the average time

New code to grow and shrink buffers guarantees that the average time
of inserting or deleting each character is bounded by a constant.  The
user is given some control over this behavior by means of the editor
variable BUFFER-REALLOCATION-FACTOR, which allows a space-time
tradeoff between the average insertion/deletion time and the average
load factor of the buffer.  BUFFER-REALLOCATION-FACTOR may be given a
local value in a given buffer to change that buffer's allocation
without affecting other buffers.