internal/number: increase the max integer digits

This allows int64s to convert without allocation.

Change-Id: I6f1f95cb7cf72384ce9d420e05a077e378ebd7ad
Reviewed-on: https://go-review.googlesource.com/46156
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>

internal/number/decimal.go

diff --git a/internal/number/decimal.go b/internal/number/decimal.go
index 4e42ec7..a323ae7 100644
--- a/internal/number/decimal.go
+++ b/internal/number/decimal.go
@@ -34,6 +34,8 @@
 	Scale     int32 // maximum number of decimals after the dot.
 }
 
+const maxIntDigits = 20
+
 // A Decimal represents floating point number represented in digits of the base
 // in which a number is to be displayed. Digits represents a number [0, 1.0),
 // and the absolute value represented by Decimal is Digits * 10^Exp.
@@ -53,7 +55,7 @@
 	Inf    bool // Takes precedence over Digits and Exp.
 	NaN    bool // Takes precedence over Inf.
 
-	buf [10]byte
+	buf [maxIntDigits]byte
 }
 
 // normalize retuns a new Decimal with leading and trailing zeros removed.
@@ -401,11 +403,10 @@
 }
 
 func (d *Decimal) fillIntDigits(x uint64) {
-	const maxUintDigits = 10
-	if cap(d.Digits) < maxUintDigits {
+	if cap(d.Digits) < maxIntDigits {
 		d.Digits = d.buf[:]
 	} else {
-		d.Digits = d.buf[:maxUintDigits]
+		d.Digits = d.buf[:maxIntDigits]
 	}
 	i := 0
 	for ; x > 0; x /= 10 {