Impl WeightTrader for tuple (#3601)

* Impl WeightTrader for tuple.

* fmt

* Renaming.

* add tracing for buy_weight

* Add comment clarifying the default behavior of a WeightTrader tuple

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>
Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>