The goal of this ticket is to be able to do something like:

Hash.from_bson({a: BigDecimal('42')}.to_bson)

... and have that return ...

{a: BigDecimal('42')}

... as originally composed.

This might require a global override or flag to make Decimal128 values deserialize into BigDecimal objects (since currently those are deserialized into BSON::Decimal128 objects). We discussed having such an override in a separate file that must be manually `require`d by user applications.