all: convert ' to ′ and '' to ′′

This is necessary because gofmt in go1.19 imposes smart quotes on comments
that use pairs of single quotes. Doubled-up single tick, U+2032, is chosen
over double tick, U+2033, since the latter is harder to distinguish in many
fonts at normally used font sizes, sometimes being indistinguishable from
other superscript marks such as asterisk. Comparison: ′ ″ *

diff/fd/example_test.go

@@ -17,14 +17,14 @@ func ExampleDerivative() {
 		return math.Sin(x)
 	}
 	// Compute the first derivative of f at 0 using the default settings.
-	fmt.Println("f'(0) ≈", fd.Derivative(f, 0, nil))
+	fmt.Println("f′(0) ≈", fd.Derivative(f, 0, nil))
 	// Compute the first derivative of f at 0 using the forward approximation
 	// with a custom step size.
 	df := fd.Derivative(f, 0, &fd.Settings{
 		Formula: fd.Forward,
 		Step:    1e-3,
 	})
-	fmt.Println("f'(0) ≈", df)
+	fmt.Println("f′(0) ≈", df)
 
 	f = func(x float64) float64 {
 		return math.Pow(math.Cos(x), 3)
@@ -38,12 +38,12 @@ func ExampleDerivative() {
 		OriginKnown: true,
 		OriginValue: f(0),
 	})
-	fmt.Println("f''(0) ≈", df)
+	fmt.Println("f′′(0) ≈", df)
 
 	// Output:
-	// f'(0) ≈ 1
-	// f'(0) ≈ 0.9999998333333416
-	// f''(0) ≈ -2.999999981767587
+	// f′(0) ≈ 1
+	// f′(0) ≈ 0.9999998333333416
+	// f′′(0) ≈ -2.999999981767587
 }
 
 func ExampleJacobian() {