unionOfDisjoint: use builtins.intersectAttrs
unionOfDisjoint: use builtins.intersectAttrs

This brings two benefits:

1. The complete list of collisions is printed in the whenever any colliding
   attribute is accessed.

2. The sets are intersected using a C++ primitive, which runs in O(n) time
   (intersecting pre-sorted lists) with small constants rather than interpreted
   Nix code.

Thanks to @toonn for prompting this improvement.

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