message: use localized number formatting

This change implements default localized number
formattting for base-type arguments.
Tailored formatting, like engineering notation,
is planned through the number package.

Change-Id: I58f9c3b5543b627ab8b705a0b32819fdf317754b
Reviewed-on: https://go-review.googlesource.com/46472
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>

Author: mpvl

internal/number/format.go

@@ -454,8 +454,10 @@ func appendAffix(dst []byte, f *Formatter, affix string, neg bool) []byte {
 		case r == '-' || r == '+':
 			if neg {
 				dst = append(dst, f.Symbol(SymMinusSign)...)
-			} else {
+			} else if f.Flags&ElideSign == 0 {
 				dst = append(dst, f.Symbol(SymPlusSign)...)
+			} else {
+				dst = append(dst, ' ')
 			}
 		default:
 			dst = append(dst, string(r)...)

internal/number/pattern.go

@@ -56,6 +56,7 @@ type Pattern struct {
 	Flags        PatternFlag
 
 	// Number of digits.
+	// TODO: consider using uint32
 	MinIntegerDigits     uint8
 	MaxIntegerDigits     uint8
 	MinFractionDigits    uint8
@@ -90,6 +91,7 @@ type PatternFlag uint8
 
 const (
 	AlwaysSign PatternFlag = 1 << iota
+	ElideSign              // Use space instead of plus sign. AlwaysSign must be true.
 	AlwaysExpSign
 	AlwaysDecimalSeparator
 	ParenthesisForNegative // Common pattern. Saves space.