Work around for building with webpack5

[nokotan]

This pr will be solution to #2343.
In this PR, binding functions are declared with "function" instead of "const".

Hansihe showed:

((module, __webpack_exports__, __webpack_require__) => {
    module.exports = async () => {
        [...]
        __webpack_require__.d(__webpack_exports__, {
            [...],
            "__wbindgen_throw": () => __wbindgen_throw,
        });
        [...]

        var _my_rust_bg_wasm__WEBPACK_IMPORTED_MODULE = __webpack_require__(119);
        _my_rust_bg_wasm__WEBPACK_IMPORTED_MODULE = await Promise.resolve(_my_rust_bg_wasm__WEBPACK_IMPORTED_MODULE);

        [...]

        const __wbindgen_throw = function([..]) { [..] };

        [...]
        return __webpack_exports__;
    }();
})

Webpack tries to export "__wbindgen_throw", but "__wbindgen_throw" is undefined because "__wbindgen_throw" is declared below with "const" and undefined will be exported.

        __webpack_require__.d(__webpack_exports__, {
            [...],
            "__wbindgen_throw": () => __wbindgen_throw,
        });

Now webpack start to load .wasm, but imported object "__wbindgen_throw" is undefined, which leads silent promise rejection.

        var _my_rust_bg_wasm__WEBPACK_IMPORTED_MODULE = __webpack_require__(119);

"__wbindgen_throw" will be hoisted and it is not "undefined" when webpack exports "__wbindgen_throw".

        __webpack_require__.d(__webpack_exports__, {
            [...],
            "__wbindgen_throw": () => __wbindgen_throw,
        });
        [...]
        function __wbindgen_throw([..]) { [..] };

[alexcrichton]

Sorry for the delay in getting to this but thanks for the PR!