Fix staticcheck findings (#262)

Address some minor issues flagged by staticcheck.
None of these affect the correctness of the package.

cmp/compare_test.go

@@ -657,8 +657,8 @@ func comparerTests() []test {
 		reason:    "all zero map entries are ignored (even if missing)",
 	}, {
 		label:     label + "/PanicUnexportedNamed",
-		x:         namedWithUnexported{},
-		y:         namedWithUnexported{},
+		x:         namedWithUnexported{unexported: "x"},
+		y:         namedWithUnexported{unexported: "y"},
 		wantPanic: strconv.Quote(reflect.TypeOf(namedWithUnexported{}).PkgPath()) + ".namedWithUnexported",
 		reason:    "panic on named struct type with unexported field",
 	}, {
@@ -1279,11 +1279,11 @@ using the AllowUnexported option.`, "\n"),
 	}, {
 		label: label + "/LargeMapKey",
 		x: map[*[]byte]int{func() *[]byte {
-			b := make([]byte, 1<<20, 1<<20)
+			b := make([]byte, 1<<20)
 			return &b
 		}(): 0},
 		y: map[*[]byte]int{func() *[]byte {
-			b := make([]byte, 1<<20, 1<<20)
+			b := make([]byte, 1<<20)
 			return &b
 		}(): 0},
 		reason: "printing map keys should have some verbosity limit imposed",

cmp/example_test.go

@@ -98,9 +98,9 @@ func ExampleOption_equalNaNs() {
 		return (math.IsNaN(x) && math.IsNaN(y)) || x == y
 	})
 
-	x := []float64{1.0, math.NaN(), math.E, -0.0, +0.0}
-	y := []float64{1.0, math.NaN(), math.E, -0.0, +0.0}
-	z := []float64{1.0, math.NaN(), math.Pi, -0.0, +0.0} // Pi constant instead of E
+	x := []float64{1.0, math.NaN(), math.E, 0.0}
+	y := []float64{1.0, math.NaN(), math.E, 0.0}
+	z := []float64{1.0, math.NaN(), math.Pi, 0.0} // Pi constant instead of E
 
 	fmt.Println(cmp.Equal(x, y, opt))
 	fmt.Println(cmp.Equal(y, z, opt))
@@ -216,7 +216,7 @@ func ExampleOption_sortedSlice() {
 type otherString string
 
 func (x otherString) Equal(y otherString) bool {
-	return strings.ToLower(string(x)) == strings.ToLower(string(y))
+	return strings.EqualFold(string(x), string(y))
 }
 
 // If the Equal method defined on a type is not suitable, the type can be