ds: add perf stat parser

We can only read one output file by now, located at:
.garlic/perf.csv

garlic/ds/perf/stat.nix

@@ -0,0 +1,23 @@
+{
+  stdenv
+, python3
+, gzip
+}:
+
+resultTree:
+
+stdenv.mkDerivation {
+  name = "perf-stat.json.gz";
+  preferLocalBuild = true;
+  src = ./stat.py;
+  phases = [ "unpackPhase" "installPhase" ];
+
+  unpackPhase = ''
+    cp $src stat.py
+  '';
+
+  buildInputs = [ python3 gzip ];
+  installPhase = ''
+    python stat.py ${resultTree} | gzip > $out
+  '';
+}