nixd/Eval: basic support for value metadata #3064

Job	Run time
nix-build-develop-clang (none, release, true)	2m 59s
nix-build-develop-gcc (none, release, true)	3m 1s
nix-build-develop-clang (none, release, false)	2m 57s
nix-build-develop-gcc (none, release, false)	3m 1s
nix-build-develop-clang (none, debug, true)	2m 42s
nix-build-develop-gcc (none, debug, true)	2m 58s
nix-build-develop-clang (none, debug, false)	2m 44s
nix-build-develop-gcc (none, debug, false)	3m 2s
nix-build-develop-gcc (address, release, true)	4m 33s
nix-build-develop-gcc (address, release, false)	4m 39s
nix-build-develop-gcc (address, debug, true)	3m 50s
nix-build-develop-gcc (address, debug, false)	3m 57s
nix-build-develop-gcc (undefined, release, true)	4m 15s
nix-build-develop-gcc (undefined, release, false)	4m 11s
nix-build-develop-gcc (undefined, debug, true)	3m 18s
nix-build-develop-gcc (undefined, debug, false)	3m 23s
	55m 30s

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