Turn methods that don't store the result in their receiver into
    functions in order to preserve the convention.
  Re-jig Exp and Div by moving their guts into nat.go.
  Add ProbablyPrime to perform Miller-Rabin primality tests.
crypto/rsa: reenable key generation since we now have ProbablyPrime.

R=gri
CC=go-dev
http://codereview.prom.corp.google.com/1024038