Give allow/disallow reason in a big long tables (see #354)

[mozfreddyb]

This builds on the fixes in #351 and the suggestion I made in #354.

I'm not entirely sure a long table is the best idea, but I also don't like any of the other options :)

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[otherdaniel]

I'd really like to have that list. Particularly in light of #349, where it turns out that we omitted something because of an oversight, and having a list which records the reasons allowed us to check that it was indeed an oversight. I wonder whether we should/could generate it; both for easier spec maintenance as well as for more flexible presentation.

I like how this enumerates the set of reasons. These basically form an enum of allowability. If we keep a file with name, namespace, allowability, we could quite easily generate these list. We could also derive the built-ins from them, or at least check them against the builtins for error checking.

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Currently, we group by namespace, then list element/attr name -> allow-status. I suspect it'd be more compact if we'd group by namespace; then group by allow-status; then have a box of element/attr names.