Produce a clean build output for the causality export

[LinaUr]

Currently, when building an image with -H:+PrintCausalityGraph, the results will just be created as as set of individual files in the working directory:

methods.txt
types.txt
typeflows.txt
direct_invokes.bin
typestates.bin
interflows.bin
declaring_types.bin
typeflow_filters.bin
typeflow_methods.bin

In order to use the causality data in our observatory, the user currently has to zip these themselves and also include the reachability.json in this zip.

• Produce a single .cg.zip file
• Register it via the usual graal build output API, so a message gets printed
• Remove unneeded files declaring_types.bin and typeflows.txt
• Also integrate the reachability.json from the ReachabilityExporter-Feature in the .cg.zip-file
• Optional: Think about choosing a more efficient data structure for typestates.bin, which currently consists of >99% zeros