Cert loading from PEM & restructuring

- Added cert loading

- Extracted X509 tests

- Extracted `hash_str_to_vec` function into `utils.rs` as it is used
  in a couple of tests now

- Fixed `hash_str_to_vec`, funny enough prev test with it was working
  because I’ve made another mistake by providing VerifyNone instead of
  VerifyPeer and those 2 mistakes compensated each other. Since
  `hash_str_to_vec` is private there was no errors on client code
Not yet, but I hope once #84 lands it will be much easier to do.