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This adds a `Component` helper type and a `ForEach` helper method.

The first attempt used an interface but interfaces imply allocation. We really
can't afford to allocate here.

This will perform all the correct checks *except* that it won't bother actually
stringifying the multiaddr. It should be significantly faster.

This is a huge performance hit. Really, we just need to tell users not to modify
the result.

Also, get rid of an unnecessary pointer indirection (no api change).