a pointer to a Value, rather than the Value directly. This improves
the effectiveness of garbage collection a lot: if the Value is
stored inside the set directly, then any live pointer to the Value
causes all other attributes in the set to be live as well.
values. This improves sharing and gives another speed up.
Evaluation of the NixOS system attribute is now almost 7 times
faster than the old evaluator.
efficiently. The symbol table ensures that there is only one copy
of each symbol, thus allowing symbols to be compared efficiently
using a pointer equality test.
