strconv: Implement Ryū algorithm for shortest float printing

A lot of internal functions are exposed in export_test.go
in order to test and bench various levels of function calls.

Random tests have been run on a few billion values.

Implements golang#15672.

Change-Id: I028faa4f97c38f51709469f7314bfd7ec12f06dd

src/strconv/export_test.go

@@ -8,3 +8,47 @@ var (
 	BitSizeError = bitSizeError
 	BaseError    = baseError
 )
+
+type ExtFloat = extFloat
+type ExtFloat128 = extfloat128
+type ShortDecimal = decimalSlice
+type Decimal = decimal
+
+var Float64info = float64info
+
+var BigFtoa = bigFtoa
+var RoundShortest = roundShortest
+
+func NewShortDecimal() decimalSlice {
+	var buf [32]byte
+	var d decimalSlice
+	d.d = buf[:]
+	return d
+}
+
+func ToShort(d *decimal) decimalSlice {
+	sd := NewShortDecimal()
+	copy(sd.d[:], d.d[:])
+	sd.nd, sd.dp = d.nd, d.dp
+	return sd
+}
+
+func (d1 *ShortDecimal) Equals(d2 *ShortDecimal) bool {
+	if d1.nd != d2.nd && d1.dp != d2.dp {
+		return false
+	}
+	for i := 0; i < d1.nd; i++ {
+		if d1.d[i] != d2.d[i] {
+			return false
+		}
+	}
+	return true
+}
+
+func ShowDecimal(d *decimalSlice) string {
+	if d.nd == 0 {
+		return "0"
+	}
+	exp := d.dp - 1
+	return string(d.d[0]) + "." + string(d.d[1:d.nd]) + "e" + Itoa(exp)
+}

src/strconv/extfloat.go

@@ -124,6 +124,7 @@ var powersOfTen = [...]extFloat{
 	{0x9e19db92b4e31ba9, 1013, false},  // 10^324
 	{0xeb96bf6ebadf77d9, 1039, false},  // 10^332
 	{0xaf87023b9bf0ee6b, 1066, false},  // 10^340
+	{0x82c730bec1cac960, 1093, false},  // 10^348
 }
 
 // floatBits returns the bits of the float64 that best approximates