Implement on-demand memory initialization for the uffd feature.

This commit implements copying paged initialization data upon a fault of a
linear memory page.

If the initialization data is "paged", then the appropriate pages are copied
into the Wasm page (or zeroed if the page is not present in the
initialization data).

If the initialization data is not "paged", the Wasm page is zeroed so that
module instantiation can initialize the pages.

crates/wasmtime/src/instance.rs

@@ -522,11 +522,7 @@ impl<'a> Instantiator<'a> {
             // initialization is successful, we need to keep the instance alive.
             let instance = self.store.add_instance(instance, false);
             allocator
-                .initialize(
-                    &instance.handle,
-                    config.features.bulk_memory,
-                    &compiled_module.data_initializers(),
-                )
+                .initialize(&instance.handle, config.features.bulk_memory)
                 .map_err(|e| -> Error {
                     match e {
                         InstantiationError::Trap(trap) => {