Refactor trep-engine.scm for V6

[flywire]

With discussion of development moving towards V6, it would be a good time
to discuss refactoring trep-engine.scm. As it stands, the Transaction
Report engine is a massive, highly complex monolith that is very difficult
to maintain, debug, or extend.

I'd like to see it reworked with more helper modules.

Perhaps:
* Query & Filtering
* Grid Matrix Processing
* Subtotaling & Calculations

Others are far more familiar with this report than me, but I think breaking
the report into simpler modules would make it more workable.

Regards
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