archive,cmd,images: clean up some code

Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com>

archive/compression/compression.go

@@ -60,7 +60,7 @@ func DetectCompression(source []byte) Compression {
 			// Len too short
 			continue
 		}
-		if bytes.Compare(m, source[:len(m)]) == 0 {
+		if bytes.Equal(m, source[:len(m)]) {
 			return compression
 		}
 	}

archive/compression/compression_test.go

@@ -53,7 +53,7 @@ func testCompressDecompress(t *testing.T, size int, compression Compression) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if bytes.Compare(orig, decompressed) != 0 {
+	if !bytes.Equal(orig, decompressed) {
 		t.Fatal("strange decompressed data")
 	}
 }