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This PR is a non-substantive change with the goal of making this method
`ExtractPublicKey` from a `peer.ID` clearer to the reader. This goal is
accomplished by using the full name of the identity hash code `IDENTITY` instead
of the deprecated `ID` code. This change makes its clearer to the reader that
there is a trivial way to compute the public key given the digest. Without this
change it is easy to confuse `ID` with a concept relating to peer IDs (and thus
implying this function somehow computes a hash pre-image to get the
corrresponding public key.