Add comparative performance benchmarks

Includes a 'css-modules' implementation of the nested trees to act as a
baseline for comparison.

performance/tests/renderDeepTree.js

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+import createRenderBenchmark from '../modules/createRenderBenchmark';
+import NestedTree from '../modules/NestedTree';
+import React from 'react';
+
+const renderDeepTree = (label, components) => createRenderBenchmark({
+  name: `Deep tree [${label}]`,
+  runs: 20,
+  getElement() {
+    return (
+      <NestedTree
+        breadth={3}
+        components={components}
+        depth={6}
+        id={0}
+        wrap={1}
+      />
+    );
+  }
+});
+
+export default renderDeepTree;

performance/tests/renderWideTree.js

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+import createRenderBenchmark from '../modules/createRenderBenchmark';
+import NestedTree from '../modules/NestedTree';
+import React from 'react';
+
+const renderWideTree = (label, components) => createRenderBenchmark({
+  name: `Wide tree [${label}]`,
+  runs: 20,
+  getElement() {
+    return (
+      <NestedTree
+        breadth={10}
+        components={components}
+        depth={3}
+        id={0}
+        wrap={4}
+      />
+    );
+  }
+});
+
+export default renderWideTree;