Explain macro to handle metadata for descibing the vdbe instructions.

[Cluster444]

This includes p1, p2 and p3 mapped from a match table based on attribute tags added to the enum variant fields.

A description attribute that is pulled from the enum variant and builds out a const array for the .opcodes command that is generated from the descriptions.

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I just saw that #528 was overlapping this one. So I thought I would at least get this up to discuss and try to mitigate as many merge conflicts as possible.

I have more coming for this, but I had to stash the p4, p5 pieces so I could get this up before there was more overlap.