Add a fast track to d.Cmp(d2) when d.exp == d2.exp

This makes a noticeable difference when one is sorting a large slice
of decimals when most of them have the same precision, as it is saving
a lot of calls to rescale().

decimal.go

@@ -294,6 +294,13 @@ func (d Decimal) Div(d2 Decimal) Decimal {
 //     +1 if d >  d2
 //
 func (d Decimal) Cmp(d2 Decimal) int {
+	d.ensureInitialized()
+	d2.ensureInitialized()
+
+	if d.exp == d2.exp {
+		return d.value.Cmp(d2.value)
+	}
+
 	baseExp := min(d.exp, d2.exp)
 	rd := d.rescale(baseExp)
 	rd2 := d2.rescale(baseExp)

decimal_test.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"encoding/xml"
 	"math"
+	"sort"
 	"strconv"
 	"strings"
 	"testing"
@@ -895,3 +896,19 @@ func didPanic(f func()) bool {
 	return ret
 
 }
+
+type DecimalSlice []Decimal
+
+func (p DecimalSlice) Len() int           { return len(p) }
+func (p DecimalSlice) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }
+func (p DecimalSlice) Less(i, j int) bool { return p[i].Cmp(p[j]) < 0 }
+func Benchmark_Cmp(b *testing.B) {
+	decimals := DecimalSlice([]Decimal{})
+	for i := 0; i < 1000000; i++ {
+		decimals = append(decimals, New(int64(i), 0))
+	}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		sort.Sort(decimals)
+	}
+}