Add generator-specific context

[Simn]

This PR adds a lightweight version of Common.context which holds all the data generators need. For now it's only implemented for genjvm, but should be relatively easy to add to other generators.

The main goal is to make my dependency dot graphs look nicer.

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[Simn]

I have also already "ported" our second best generator genhl, which was easy enough.

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Unlike in some other areas, smaller is better.