Concatenates multiple async iterables into one async iterable.

import { concat } from "./concat.ts";

async function* gen() { yield "foo"; yield "bar"; yield "baz"; yield "qux" }
const iterable = concat(gen(), ["a", "b", "c", "d"]);
for await (const value of iterable) console.log(value);

The above example will print the following 8 lines:

foo
bar
baz
qux
a
b
c
d