When marshalling Go types to BSON, users can choose between Marshal(), which assumes the value is a document, and MarshalValue(), which works for all values and returns both a type and a slice of bytes.

For going from BSON bytes to Go, the Unmarshal() function mirrors Marshal() and assumes the input is a document. If the input is some other value like an array, users currently need to do:

val := bson.RawValue{
    Type: bsontype.Array, // or whatever the type is
    Value: bsonBytes,
}
var unmarshalled bson.A
if err := val.Unmarshal(&unmarshalled); err != nil {
   ...
}

We should consider adding a UnmarshalValue() function to mirror MarshalValue().

Definition of done:

• Add a function bson.UnmarshalValue that, given a BSON type and a byte slice, unmarshals the BSON value into a Go value. Its signature should mirror the bson.MarshalValue API.
  For example:

  func UnmarshalValue(t bsontype.Type, data []byte, val interface{}) error