msgpack: add string() for decimal, datetime. interval

Added a benchmark, which shows that the code for decimal is optimized two or more times for string conversion than the code from the library. Added a datetime, Interval type conversion function to a string, added tests for this function.

Added #322

Author: babyTsakhes

decimal/decimal.go

@@ -214,7 +214,8 @@ func (d Decimal) stringFromInt64(value int64, scale int) string {
 		// Numbers like 0.00123.
 		// Count trailing zeros in the fractional part.
 		trailingZeros := 0
-		// In this case, the fractional part consists of (scale-length) zeros followed by the number.
+		// In this case, the fractional part consists
+		//  of (scale-length) zeros followed by the number.
 		// We need to count trailing zeros in the actual number part.
 		for i := length - 1; i >= 0 && str[i] == '0'; i-- {
 			trailingZeros++