Make API more consistent

[FSMaxB]

Currently different functions arrange their parameters without any apparent system as how to arrange them consistently. Some rules are required to make this consistent.

It might be useful to follow the conventions that libsodium follows.

Some suggestions:

• output first, followed by input
• public key first, then private key
• optional parameters at the end (optional outputs after regular inputs)
• strings/data are followed by their length (when not using buffer_t)
• if a function works on a struct, pass the pointer first (like a self parameter for class methods)
  • same for IDs that represent some object

Naming conventions:

• Functions:
  • molch_end_conversation instead of molch_conversation_end (verb first)
  • but start with the 'class' that the function belongs to (molch in the example above)

[FSMaxB]

Most of that happened in the C++ port.