Use Git submodules for Krakatau and enjarify

Currently Krakatau and enjarify are included as ZIP files (multiple times).

It might be better to use Git submodules instead. This would have the following advantages:

• It is clearer which version is included.
  The current versioning uses version numbers which do not exist for that repository, e.g. Krakatau-11.zip; Krakatau does not seem to have any release versions or Git tags. Additionally the ZIP files do not seem to include information about which exact commit they contain.
• They increase trust.
  I don't want to allege any bad intentions, but especially for new users of this project it would increase trust when they directly see which version of Krakatau and enjarify is included, instead of having to manually find out if the ZIP files are legit. This also makes it for the maintainers easier to see if pull requests from new contributors claiming to update these ZIP files have bad intentions.
• It decreases the repository size.
  Instead of having to replace a binary ZIP file for every version update, only the Git submodule data has to be updated.

This might also make it easier to omit certain directories when building the bytecode-viewer JAR, such as the test, examples and documentation directories of Krakatau and enjarify (if their licenses permits this).

Start by locating the repeated Krakatau and enjarify ZIP files and the process that packages them into the bytecode-viewer JAR. Review the submodule workflow and verify the licenses before deciding whether test, examples, and documentation directories can be omitted. Done means the dependencies are referenced at exact commits, repository size is reduced, and the resulting JAR still builds correctly.