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The latter isn't used yet; that's not the main goal of this branch (and
I think I'd rather do the refactor to use ipld.ErrNotExists after
moving the PathSegment types from the selector package into the root...
which in turn I'm not going to do while some of the other current work
on Selectors is in progress in other branches.  So!  Latur.); I just
wanted it around for documentation and clarity of intent.

This also adds a couple of other dummy references to the 'typed'
package.  I know keeping a set of packages-used-in-this-file is a
common step in the development of codegen systems targetting golang,
but I'm going to try to avoid it until the need is really forced.