stream: adding a benchmark

Based on stdlib's `sha256`. Initial output:
BenchmarkHash8Bytes       100000             20387 ns/op           0.39 MB/s
BenchmarkHash1K    50000             25876 ns/op          39.57 MB/s
BenchmarkHash8K    50000             38836 ns/op         210.94 MB/s

stream_test.go

@@ -136,3 +136,28 @@ func TestMerkleHashWriter(t *testing.T) {
 	}
 
 }
+
+var bench = NewHash(DefaultHashMaker, 8192)
+var buf = make([]byte, 8192)
+
+func benchmarkSize(b *testing.B, size int) {
+	b.SetBytes(int64(size))
+	sum := make([]byte, bench.Size())
+	for i := 0; i < b.N; i++ {
+		bench.Reset()
+		bench.Write(buf[:size])
+		bench.Sum(sum[:0])
+	}
+}
+
+func BenchmarkHash8Bytes(b *testing.B) {
+	benchmarkSize(b, 8)
+}
+
+func BenchmarkHash1K(b *testing.B) {
+	benchmarkSize(b, 1024)
+}
+
+func BenchmarkHash8K(b *testing.B) {
+	benchmarkSize(b, 8192)
+}