The memoize function is not a pretty thing in terms of implementation,
but it's exactly what we need to solve this UX problem and performance
problem.
Without this, every distinct `withSystem` call will cause a re-evaluation
of the `perSystem` module, which is inefficient. Now, it's a one-time
13KB and length(system) attribute lookups: negligible compared to any
instantiations and such.
Nix doesn't offer memoization for functions yet, so this is the best we
can do.
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