Benchmark existing ways to check for IDENTITY CIDs

Add benchmarks that compare two ways of checking for
`multihash.IDENTITY` code:
1. `Cid.Prefix().MhType`
2. Decode of `Cid.Hash()`

This benchmark illustrates that using Cid.Prefix is more efficient than
`multihash.Decode`. Users wishing to perform such a check should use
`Cid.Prefix`.

Consider that `Cid.Prefix` is already efficient enough and introducing a
dedicated API for performing this check will likely result in small
gains.

Relates to #133

benchmark_test.go

@@ -0,0 +1,61 @@
+package cid_test
+
+import (
+	"math/rand"
+	"testing"
+
+	"github.com/ipfs/go-cid"
+	"github.com/multiformats/go-multihash"
+)
+
+// BenchmarkIdentityCheck benchmarks two ways of checking whether a CIDv1 has multihash.IDENTITY
+// code:
+//  1. Cid.Prefix(), and
+//  2. decoding the Cid.Hash().
+//
+// This benchmark illustrates that using Cid.Prefix is more efficient than multihash.Decode.
+// Users wishing to perform such a check should use Cid.Prefix.
+//
+// Consider that `Cid.Prefix` is already efficient enough and introducing a dedicated API for
+// performing this check will likely result in small gains.
+func BenchmarkIdentityCheck(b *testing.B) {
+	rng := rand.New(rand.NewSource(1413))
+
+	data := make([]byte, rng.Intn(100)+1024)
+	if _, err := rng.Read(data); err != nil {
+		b.Fatal(err)
+	}
+	mh, err := multihash.Sum(data, multihash.IDENTITY, -1)
+	if err != nil {
+		b.Fatal(err)
+	}
+	cv1 := cid.NewCidV1(cid.Raw, mh)
+
+	b.SetBytes(int64(cv1.ByteLen()))
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	b.Run("Prefix", func(b *testing.B) {
+		b.RunParallel(func(pb *testing.PB) {
+			for pb.Next() {
+				if cv1.Prefix().MhType != multihash.IDENTITY {
+					b.Fatal("expected IDENTITY CID")
+				}
+			}
+		})
+	})
+
+	b.Run("MultihashDecode", func(b *testing.B) {
+		b.RunParallel(func(pb *testing.PB) {
+			for pb.Next() {
+				dmh, err := multihash.Decode(cv1.Hash())
+				if err != nil {
+					b.Fatal(err)
+				}
+				if dmh.Code != multihash.IDENTITY {
+					b.Fatal("expected IDENTITY CID")
+				}
+			}
+		})
+	})
+}