This is necessary for exported Scala classes to be instantiable
with `new ExportedClass()`.

The previous strategy for top-level export could not lead to any
workable support of mutable vars. We now follow a strategy similar
to what the JS backend uses in its NoModule configuration.

Previously, we compiled top-level exports as Wasm global exports.
We then had a postprocessing step in the loader to extract the
`.value` of the `WAGlobal` instances. This correctly captured the
exported value right after the start function has finished.
However, it could not update the exported value after a mutable
static field was reassigned.

Now, we turn everything everything around. Instead of the JS loader
and wrapper *pulling* values from Wasm, the Wasm code *pushes*
updates to top-level exports to JavaScript. That means we declare
setter functions from JavaScript, and that we (counter-intuitively)
*import* those setters into Wasm. Wasm calls the setters in the
start function for all top-level exports, and again when assigning
static fields that are exported.