big: use fast shift routines

big: use fast shift routines

- fixed a couple of bugs in the process
  (shift right was incorrect for negative numbers)
- added more tests and made some tests more robust
- changed pidigits back to using shifts to multiply
  by 2 instead of add

  This improves pidigit -s -n 10000 by approx. 5%:

  user 0m6.496s (old)
  user 0m6.156s (new)

R=rsc
CC=golang-dev
https://golang.org/cl/963044