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A bit fun; wish there was a way to compress that block of ifdefs about
optional vs nullable stuff, but so far nothing clearer has emerged.

Added ErrIteratorOverread while at it.  Should refactoring other node
types to use it too, I suppose.  But perhaps we'll just accumlate those
todos for a while, since there's already one other error refactor with
a chain of blocks in front of it.

Exercise unmarshal against our generated "Strang" type.  Works!

(Note: this will fail if you *haven't* run the codegen yet; but it's in
an underscore-prefixed package, so it won't run unless you dive in here
intentionally.  Overall, this continues to be early-days hijinx, and
does not represent a good long-term testing strategy.)