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I'm still not fully convinced this is always a win for performance,
but as discussed in
https://github.com/ipld/go-ipld-prime/pull/143#discussion_r582335774 ,
there was past evidence, and reproducing that takes work -- so, we're
going to try to be relatively conservative here and keep this logical
branch behavior in place for now.

(The reason this may be worth re-examining is that the change to
hashing interface probably gets rid of one big source of copies.
That doens't answer the holistic performance question alone, though;
it just reopens it.)