Refactor the macro tables in Buffer.

Refactor the macro tables in Buffer.

The use of maps of maps of structs is beyond confusing. Replace that
with a class that hides at least some of the complexity. There is
probably more that could be done along the same lines.

(Note: This fixes a thinko but is otherwise the same as the previous
commit. So it rewrites history.)

(cherry picked from commit dbf24b113ebb559d91b20fd84c7de1b96684d752)