Improve messages module

[leoschwarz]

• [x] Move into its own crate since its big and generally will not change that much but seriously slows down compilation of everything else (at least until incremental compilation lands in stable rustc – or before doing this move try out the incremental compilation → https://internals.rust-lang.org/t/incremental-compilation-beta/4721).
• [x] Start using the documentation from opensim-protocol. For this the code generator will have to be updated to always check if there are entries in opensim-protocol and only fall back to the old way of creating the files if there isn't. In the far future we want to only use the xml files as a basis.