Read Reader r until EOF (null) and resolve to the content as Uint8Array`.

// Example from stdin
const stdinContent = await readAll(Deno.stdin);

// Example from file
const file = await Deno.open("my_file.txt", {read: true});
const myFileContent = await readAll(file);
Deno.close(file.rid);

// Example from buffer
const myData = new Uint8Array(100);
// ... fill myData array with data
const reader = new Buffer(myData.buffer);
const bufferContent = await readAll(reader);