Inconsistent MP serialization between decimal impls

We now have two decimals formats available, they serialize the same decimal into two different ways. Taking this decimal as an example: "3333.3333". MP serialize inconsistency can be visually seen on . Left one is older version - through rust libdec, right one is a new format.

was:

Ext(
                        1,
                        [
                            210,
                            0,
                            0,
                            0,
                            4,
                            3,
                            51,
                            51,
                            51,
                            60,
                        ],
                    )

become:

Ext(
                        1,
                        [
                            4,
                            3,
                            51,
                            51,
                            51,
                            60,
                        ],
                    )