The bson crate supports round-trip deserialization/serialization of decimal128 values, but only supports creating or inspecting the value as a raw byte array. Per the spec, we need to add support for constructing a Decimal128 from a human-readable string and producing human-readable strings from existing values.

An incomplete implementation of decimal128 logic already exists at https://github.com/mongodb-rust/decimal128 and likely will be very useful for this.