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And add a bunch of type-specific ones for helpfulness.

But immediately re-reviewing this further.  What we've done here is
handle leafs very oddly, because it would seem to avoid unnecessary
wrapping on the leaf nodes.  But the result seems to be much uglier
code than it needs to be, and implies that we've got all sorts of
additional special cases required to either handle or reject binds
of primitive-kind fields.  And that seems... maybe not a great trade.
Signed-off-by: Eric Myhre <hash@exultant.us>