Speed up compilation, reduce disk footprint of multiple datalog programs about differential-datalog HOT 4 OPEN

I think one way to increase sharing would be to somehow use a Cargo workspace as we have done for the crates below rust/template. However, I am not sure if nesting of workspaces is possible and how to keep the workspace up-to-date with newly added tests to begin with. So it does not sound like an appealing way to go about that to me, but at the same time it's the only thought that came to mind.

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#501 made significant progress towards resolving this issue. Remaining TODOs:

• Move differential-datalog crate out of the template and into a shared location, e.g., $DDLOG_HOME, so we only need to compile it once and not for each DDlog program. This way we should be able to run more tests.
• See if we can speed-up recompilation of large programs even more, e.g., by placing each value type in a separate module that will only be recompiled when that type changes.
• Smarter memory allocation strategy that would allocate multiple values at once using domain-specific information from DD.

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More improvements in #512 and #520.

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It appears that a lot of remaining overhead comes from auto-derived trait implementations. So the next step would be to see if we can do better by getting DDlog to generate these trait implementations instead.

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