Add a good PRNG (mersenne twister) for cctools.

NOTE: this is not a CSPRNG.
Includes some useful helper functions.
Replaces the old random_init.[ch]. srand is still seeded.
Merge remote-tracking branch 'ccl/pull/573/head'

* ccl/pull/573/head:
  Add a good PRNG (mersenne twister) for cctools.