DH numbers

Probably needs some parameter validation. OpenSSL's DH_check appears to do some reasonably cheap looking tests we can copy over.

(Also docs.)
We have both of these styles in cryptography. I actually prefer the current form this PR has, but frankly I don't think we can enforce one or the other unless we have a linter for it. :( Who knew I'd want PEP8 to be more proscriptive?