Is your feature request related to a problem? Please describe.
Name-value pairs like { name = "..."; value = "..."; } are frequently used, especially when converting between attribute sets and lists using builtins.listToAttrs and lib.mapAttrsToList lib.nameValuePair. It would be beneficial for these operations to be optimized.

Describe the solution you'd like
It should be possible to implement an optimization for such values by storing the name and value directly in the Value union, just like how lists of sizes <= 2 are.

Ping @roberth

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