Support exporting Rust functions as variadic JS functions

[richard-uk1]

Support the following example

#[wasm_bindgen(apply_ending_slice_bikeshedded)]
pub fn add(first: u32, second: u32, rest: &[u32]) -> u32 {
    rest.iter().sum() + first + second
}

callable from javascript as

import { add } from './wasm';

console.log(add(1, 2, 3, 4, 5)); // prints 15

You could implement the transformation in javascript as

function add(first, second, ..rest) {
    return add_from_rust(first, second, rest);
}

This is a prerequisite for implementing #503 the reverse of #503.

[alexcrichton]

Seems plausible to me! I think though that this is the opposite of #503, right? (this is about defining something in Rust whereas that's about importing something defined in JS)

[richard-uk1]

Yeah I don't know why I said prerequisite :)

[joaofreires]

@alexcrichton maybe we can close this issue as #2954 has been merged.