`Config.Ciphers` helper module for picking ciphers
make config-creating funs more regular

Suggestions by @dbuenzli. Invalid configuration should be treated as programming
error.

- move exception to the standard `Invalid_argument`
- remove `_exn` suffix from names
remove overy aggresive config check
Merge pull request #177 from mirleft/ciphersuite-combinators

Ciphersuite combinators