refactor: use a helper type to decode AddrInfo from JSON

[Stebalien]

This saves us a bunch of error prone manual decode/encode logic.

[marten-seemann]

This works, my fuzzer (not comitted yet) doesn't find any crashes. It requires extra allocations though, not sure if we should care about that.

[Stebalien]

It requires extra allocations though, not sure if we should care about that

Really? I'd expect fewer allocations as we can avoid a lot of interface indirection.

Regardless, I don't think it will matter much either way.

[marten-seemann]

This PR:

goos: darwin
goarch: amd64
pkg: github.com/libp2p/go-libp2p-core/peer
BenchmarkAddrInfoJSON-16    	  151012	      7853 ns/op	    2787 B/op	      46 allocs/op

#177:

goos: darwin
goarch: amd64
pkg: github.com/libp2p/go-libp2p-core/peer
BenchmarkAddrInfoJSON-16    	  135700	      8511 ns/op	    3658 B/op	      65 allocs/op

benchmark code:

func BenchmarkAddrInfoJSON(b *testing.B) {
	for i := 0; i < b.N; i++ {
		ai := AddrInfo{ID: testID, Addrs: []ma.Multiaddr{maddrFull}}
		out, err := ai.MarshalJSON()
		if err != nil {
			panic(err)
		}
		var addrInfo AddrInfo
		if err := addrInfo.UnmarshalJSON(out); err != nil {
			panic(err)
		}
		if addrInfo.ID != testID {
			panic(fmt.Sprintf("expected ID to equal %s, got %s", testID.Pretty(), addrInfo.ID.Pretty()))
		}
		if len(addrInfo.Addrs) != 1 || !addrInfo.Addrs[0].Equal(maddrFull) {
			panic(fmt.Sprintf("expected addrs to match %v, got %v", maddrFull, addrInfo.Addrs))
		}
	}
}

[marten-seemann]

That's unambiguous. Let's use the PR. @Stebalien can you cherry-pick the test case I wrote for #177?