As an ergonomic improvement, we should implement From<Option<T>> for Bson for T that can be converted into Bson, returning Bson::Null in the None case.

This would allow Option to more easily be used in the doc! macro:

let x: Option<i32> = None;
// current
doc! { "x": x }; // compile error, Bson: From<Option<i32>> not satisfied

// proposed
doc! { "x": x }; // { "x": null }