Cache the result of comparer.identical for a massive speedup

[bobg]

This PR adds a memoizing cache to the comparer type for the identical method, producing a sometimes enormous speedup.

[github-actions]

Modver result

This report was generated by Modver,
a Go package and command that helps you obey semantic versioning rules in your Go module.

This PR requires (at least) an increase in your module's patchlevel.

checking newComparer
  in results of func() *github.com/bobg/modver/v2.comparer
    old and new versions of github.com/bobg/modver/v2.comparer are not identical
      Patchlevel