This avoids building the _id (group key) objects while processing inputs to
$group. Instead it just remembers the values of the fields, and saves building
the object until it is preparing the output.
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.
