populate; add doc.populated()
use better internal data name

a schema path beginning with $ is invalid in mongodb, so
we know that no user will use this
populate; doc.populated now gets/sets
populate; docs
populate; now backwards compatible

nulls and unfound documents no longer included in
populated paths (just like pre 3.6)
populate; add tests for doc.populated
populate; more tests
populate; add benchmark

3.5.5: 8211  completed
3.6x:  12358 completed