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This new system focuses on table-driven tests, and leans heavily upon
json as a shorthand for expressing fixtures.

It also makes a great deal more effort to exercise the different
features of nodes (and their paired representation nodes) from all
directions at once for each test datum, rather than requring that all
be written out manually.

The result is that the struct tests we've renovated have a lovely
diffstat shrinkage: 111 insertions, 299 deletions...

And yet the smaller line count results in *more* coverage.

(Okay, the linecount increase for the testcase structure and helper
methods is much bigger than the savings in fixture size... but,
only *so far*.  I assume this will continue to pay off in the future.)

Relatedly: a bug in struct map representations has been fixed.
(It was the sibling of 5f589653, embarassingly.)  Thank goodness we now
get proper coverage of this area.

There's a few TODOs left to further expand the exercises, but those
can slot in easily in subsequent commits.  Same goes for further
expansion of usage of this new system.