(tracking issue for https://github.com/mongodb/bson-rust/issues/134 and https://github.com/mongodb/bson-rust/pull/135)

A user has pointed out that the underlying crate we use to model documents, linked-hash-map, has been marked as being in maintenance mode and made a pull request to update our implementation to use the indexmap crate. We should investigate whether we want to move off of the linked-hash-map crate, and if so, which underlying map type we want to replace it with.