big: cleanups and performance tuning

- removed last argument (n) from all core arithmetic routines;
  instead, use the length of the result

- simplified nat.make implementation and chose a better capacity
  for new values, removed a TODO in the process

Changing the constant e from 1 (old) to 4 (new) improved
pidigits -s -n 10000 by ~9% (on a 3.06GHz Intel Core 2 Duo):

user	0m3.882s (old)
user	0m3.549s (new)

R=rsc
CC=golang-dev
http://codereview.appspot.com/1133043

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