o Code simplifications and refactoring:
    - Use our own Base64 encoder instead of OpenSSL's, to allow more control
      over the output. Part of ticket 15652.

  o Removed code:
    - Remove `USE_OPENSSL_BASE64` and the corresponding fallback code and
      always use the internal Base64 decoder. The internal decoder has been
      part of tor since tor-0.2.0.10-alpha, and no one should be using the
      OpenSSL one. Part of ticket 15652.