Add a script to check that cells are under the right headers.

[jyasskin]

Depends on #30, since that changes the structure of the header cells.

It's hard to be sure that the checks and x's keep matching their column headers as the threat-model tables change, since they're far apart in the source, so this script emits console.errors when the local declaration of the goal doesn't match the reader-visible column header.

@tabatkins, FYI in case you see an easy way to surface these errors more visibly or, even better, during the Bikeshed build.

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[tabatkins]

Hmm, so a general capability to explicitly tie cells to particular columns, to ensure order doesn't get messed up? Theoretically doable, yeah.

[jyasskin]

Now that #30 is merged, this is back to a single commit.