Index-accelerated $lookup foreign-side
v0.3.0a48 makes $lookup fast. Previously the aggregation stage did an O(N×M) nested loop — every outer doc against every foreign doc. Now the foreign side is an O(N+M) hash join: _build_lookup_index keys foreign docs by their foreignField value (with array values expanded element-wise), and _hash_join_lookup does dict lookups per outer doc. For the standard "fan-out from a small parent table to a larger child table" shape, that's the same complexity profile mongod uses when there's no index on the foreign side.
A future release will route through real foreign-side indexes when one exists; this commit is the in-memory hash-join baseline.