wasmMemory cleanup

Hi,

I have a specific case with emcripten wasm executed from within my own Worker where I want to create and execute main (using pthreads) in loop, while using 2GB of INITIAL_MEMORY. It seems the most performant to do the following:

```js
const INITIAL_MEMORY=2146435072;
const wasmMemory=new WebAssembly.Memory({"initial":INITIAL_MEMORY/65536,"maximum":4294967296/65536,"shared":true});
for (...) {
resetMemory();
module = await createModule({wasmMemory, ...});
await runMain(); // executes module._emscripten_proxy_main(...);
module.PThread.terminateAllThreads(); // destroy all module related workers, otherwise these doesnt get GCed
}
```

So instead of reallocating this large WebAssembly memory, I reuse it while resetting before each creation.

My question is, whats the fastest way / algorithm to reset the memory, considering how emscripten reads and use it, inside my `resetMemory()`? Obviously I can reset it whole which seems slow:

```js
new Uint32Array(wasmMemory.buffer).fill(0); // takes up to 900ms
```

...but it also seems sufficient to reset just first 16Mb

```js
new Uint32Array(wasmMemory.buffer).fill(0, 0, 1024 * 1024 * 4); // ~4ms
```

I am thinking there might be some particular byte, byte range or even better built-in method to do the reset?