The actual change is in src/mongo/shell/collection.js with the remaining changes
to adjust tests to the new api.
The "ideal" arguments are now an array of pipeline ops and an optional object
with extra top-level options for the command (such as {explain: true}. For
backwards compatibility, we still support the varargs style where each argument
is a pipeline stage, but there is no way to specify extra parameters in this
mode.
As fallout from the main change, ExpressionObjects now need to be told
at construction time when they should use the semantics of a root object
in a $project (special handling of _id).
