Implement `c:upsert/2` as a pre-configured version of `c:insert/2`

[joeljuca]

Ecto.Repo's c:insert/2 can be used to perform UPSERT operations (an INSERT opts that becomes UPDATE if a given entry is already present in the repository).

See: https://hexdocs.pm/ecto/Ecto.Repo.html#c:insert/2-upserts