There's a couple of instances where `Promise<void>` is still there,
those are the exceptions, for example functions that explicitely return
a `Promise` object or functions that return the promise from another
function call without any `await` statement.
used this one-liner to automatically remove everything:
```bash
grep -rli 'promise<void>' | xargs sed -i 's/: Promise<void>//g
```
* Prepend underscores to private modules
* Remove collectUint8Arrays() It would be a misuse of Deno.iter()'s result.
* Move _util/async.ts to async
* Move util/sha*.ts to hash
