(Unlikely) Fix #2769: use a positional operator for inserting L1 embedded documents.

This does pass all the specs, but the implementation feels very wrong. It should be in something like `Mongoid::Atomic::Paths::Embedded`, but I failed to find another place to insert similar path-like behavior for an insert. 

Curious to see what the proper fix would be.

This is an attempt for #2769.