Better safe integer range for numbers

This commit optimizes the safe float to int conversion function
by using the full -9007199254740991 to 9007199254740991 range as
suggested at:

https://tc39.es/ecma262/#sec-number.min_safe_integer

closes #174

gjson.go

@@ -125,16 +125,17 @@ func (t Result) Int() int64 {
 		return n
 	case Number:
 		// try to directly convert the float64 to int64
-		n, ok := floatToInt(t.Num)
-		if !ok {
-			// now try to parse the raw string
-			n, ok = parseInt(t.Raw)
-			if !ok {
-				// fallback to a standard conversion
-				return int64(t.Num)
-			}
+		i, ok := safeInt(t.Num)
+		if ok {
+			return i
 		}
-		return n
+		// now try to parse the raw string
+		i, ok = parseInt(t.Raw)
+		if ok {
+			return i
+		}
+		// fallback to a standard conversion
+		return int64(t.Num)
 	}
 }
 
@@ -150,16 +151,17 @@ func (t Result) Uint() uint64 {
 		return n
 	case Number:
 		// try to directly convert the float64 to uint64
-		n, ok := floatToUint(t.Num)
-		if !ok {
-			// now try to parse the raw string
-			n, ok = parseUint(t.Raw)
-			if !ok {
-				// fallback to a standard conversion
-				return uint64(t.Num)
-			}
+		i, ok := safeInt(t.Num)
+		if ok && i >= 0 {
+			return uint64(i)
 		}
-		return n
+		// now try to parse the raw string
+		u, ok := parseUint(t.Raw)
+		if ok {
+			return u
+		}
+		// fallback to a standard conversion
+		return uint64(t.Num)
 	}
 }
 
@@ -2525,25 +2527,11 @@ func parseInt(s string) (n int64, ok bool) {
 	return n, true
 }
 
-const minUint53 = 0
-const maxUint53 = 4503599627370495
-const minInt53 = -2251799813685248
-const maxInt53 = 2251799813685247
-
-func floatToUint(f float64) (n uint64, ok bool) {
-	n = uint64(f)
-	if float64(n) == f && n >= minUint53 && n <= maxUint53 {
-		return n, true
+func safeInt(f float64) (n int64, ok bool) {
+	if f < -9007199254740991 || f > 9007199254740991 {
+		return 0, false
 	}
-	return 0, false
-}
-
-func floatToInt(f float64) (n int64, ok bool) {
-	n = int64(f)
-	if float64(n) == f && n >= minInt53 && n <= maxInt53 {
-		return n, true
-	}
-	return 0, false
+	return int64(f), true
 }
 
 // execModifier parses the path to find a matching modifier function.