Impl Hash/Eq on BSON

[NineLord]

Hi,
I would like to be able to use BSON as key in HashMap, aka implement Hash/Eq on BSON.
I understand that it isn't a best practice to do that, due to BSON::Document and BSON::Double, but having this ability (even behind a feature flag) would help a lot.
serde_json::Value does support that too.