Address WTF-PAD comments by Nick (2018-11-27 over IRC)

Nick expressed the following concerns/comments about the branch:

• Figure out how dormant mode interacts with the padding code.

15:27 <+nickm> okay.  short version is that tor can enter a "dormant" state when it is inactive for a really long time, or when the controller tells it to do so.
15:28 <+nickm> we should probably figure out whether being dormant prevents you from sendig padding and vice versa...
15:28 <+nickm> ... and whether it should prevent you from sending padding (and vice versa)...

• Do type checking when downcasting the vcarious probability distributions.

• <+nickm> Also there are all these generic dist_ops functions that you could use ... but the API seems to encourage you not to use them. having wrapper functions instead that call dist->dist_ops.func(dist, ...) would be neat

• <+nickm> oh, one list thing: I think src/lib/math is not allowed to include lib/crypt_ops, since that would introduce a circularity. Better make sure that it doesn't