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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 showed up when profile go-ipfs.

This is checked when the protocol is *registered*. There's no reason to test it
twice.

Otherwise, .String() can panic.

Instead of having the transformers deal with variable length argument prefixes,
just do this in the bytesToString/stringToBytes functions.

Allowing multiaddr extensions to have data with