Bson provides mutable getters to for each of the variants in the enum, e.g. Bson::as_document_mut, which returns an Option<&mut Document. While these are useful for most types, they don't provide much utility for types that implement Copy (e.g. numeric types and bool), as none of the functionality on those types is implemented in methods that take references to self. To cut down on unnecessary API surface, we should remove those extraneous as_mut helpers.