Investigate JarFileEntries index entry

[philwebb]

The JarFileEntries is used as the index for a Jar file and is constructed when the file is loaded. We could probably compute it during repackage and save as an entry. This might make loading the zips faster.

[Turbots]

Ive been looking at this issue in the code and I think this should be possible to fix by adding some sort of indices file which contains the indices, offsets and hashes. If I'm not mistaken, we will have to add the indices file itself to the file as well (inception) but this could probably be calculated beforehand?

Not sure how much time this would cut off the load times of Spring Boot apps. Every millisecond counts I guess.

Also, I assume we would have to keep backwards compatibility in mind when loading the jar, so that previously packaged jars still work ofcourse?