Wasm bindings: Make Rng interface zero-copy

[matheus23]

Currently the Rng interface in wnfs-wasm looks like this:

#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen(typescript_type = "Rng")]
    pub type Rng;

    #[wasm_bindgen(method, js_name = "randomBytes")]
    pub fn get_random_bytes(this: &Rng, count: usize) -> Vec<u8>;
}

So it's implemented like this on the JS side:

/** A pseudo-random number generator */
class Rng {
  /** Returns random bytes of specified length */
  randomBytes(count: number): Uint8Array {
    const array = new Uint8Array(count);
    self.crypto.getRandomValues(array);
    return array;
  }
}

However, that'll copy the array generated inside randomBytes from the JS heap into the Wasm heap.
We can do better, if instead we defined fn get_random_bytes(this: &Rng, buffer: &mut [u8]) or fn get_random_bytes(this: &Rng, buffer: Uint8Array) or something along those lines, where the buffer to write into is provided by the Wasm side.
crypto.getRandomBytes can write into any location in zero copies :)