Add a page of microbenchmarks for wasm-bindgen

This commit starts to add a page of microbenchmarks for wasm-bindgen
which we can hopefully track and compare over time. Right now it's
primarily focused on data collection, making it easy to collect data
across a number of benchmarks for comparison. It doesn't currently do
much in the way of actually comparing the results for you (aka drawing
pretty graphs), so let's left for a future step.

It's hoped though that we can use this to track performance improvements
as well as ensuring that they work over time!

benchmarks/utils.js

@@ -0,0 +1,14 @@
+export class Lock {
+  constructor() {
+    this.lockHolder = null;
+  }
+
+  async withLock(scope) {
+    while (this.lockHolder !== null) {
+      await this.lockHolder;
+    }
+    this.lockHolder = Promise.resolve(null).then(scope);
+    await this.lockHolder;
+    this.lockHolder = null;
+  }
+}