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/globals.js

@@ -0,0 +1,5 @@
+export function jsthunk() {}
+export function add(a, b) { return a + b; }
+export class Foo {
+  bar() {}
+}