Add more Bson From<> implementations for better ergonomics.

[jcdyer]

This PR new From<> implementations to Bson to cover Vec<T>, &[T] objects, and &T (where T implements Into + Clone). Comparable impls exist in serde_json::Value, which is, I think, a good reference point for how the Bson type should operate. Having these impls in place eliminates a lot of manual cloning.

Caveat: When using this branch in the mongodb crate, there is one type inference break: Because the From<Array> implementation (== From<Vec<Bson>>) was replaced by From<Vec<T>> where T: Into<Bson>, Bson::from(Vec::new()) now complains that it needs to know the full type of Vec::new(). The problem is resolved by using Bson::Array(Vec::new()) instead of Bson::from. It also only shows up in case of a Vec of unspecified type, which means it should not affect conversion from a struct member in most cases. Again, this behavior is in line with how serde_json::Value operates.