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When I started typing this, I thought I was going to need it -- or at
least *want* it -- in the recursive bits of map value assemblers.
By the end, it turns out... no, actually, I don't.  Map value assembler
does better just embedding the (very sparse) amount of relevant logic
without abstraction in this case; and it also *couldn't* embed the
anyBuilder regardless, because the anyBuilder embeds the map assembler
which embeds it's value assembler..... Fhwooo.

As you can see in some of the comments, I was also surprised to find
that it currently shakes out that there's no demand at all for an
'anyNode' type.  There could be! -- if we had maps and lists use that
rather space-costly mechanism in exchange for fewer value allocs -- but
at present, we don't.

But even if that supposed dependency path turned out wrong, of course
we still need an 'anyBuilder' regardless -- deserialization often
starts here! -- so, here it is.

More things need to be filled in yet -- for the other scalars I haven't
bothered to stamp out in this package yet, namely -- but these will be
cookie cutter and shan't be interesting.