test(tvix/eval): Add attr merge benchmarks

Add a quick couple of benchmarks for merging attribute sets, large and
small.

Change-Id: I26940a9cf4e0d30e3d9eb07a7b8c366ca4072ca3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6286
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: grfn <grfn@gws.fyi>

tvix/eval/Cargo.toml

@@ -23,6 +23,7 @@ rev = "614e7d4641e83ab7eb321d9ac5e3d12985f9f3ba"
 criterion = "0.3.6"
 test-generator = "0.3.0"
 pretty_assertions = "1.2.1"
+itertools = "0.10.3"
 
 [features]
 # Enables running the Nix language test suite from the original C++