Create a Deno.Reader from an iterable of Uint8Arrays.

     import { readerFromIterable } from "./streams.ts";
     import { serve } from "../http/server_legacy.ts";

     for await (const request of serve({ port: 8000 })) {
       // Server-sent events: Send runtime metrics to the client every second.
       request.respond({
         headers: new Headers({ "Content-Type": "text/event-stream" }),
         body: readerFromIterable((async function* () {
           while (true) {
             await new Promise((r) => setTimeout(r, 1000));
             const message = `data: ${JSON.stringify(Deno.metrics())}\n\n`;
             yield new TextEncoder().encode(message);
           }
         })()),
       });
     }