Support Uint8Array instances as Module.stdin and as Module's stdout/stderr

Overriding Module.stdin and doing an iterator works, but is very slow for large-ish inputs:
```js
stdin()
{
if(Module.input_stdin_binary.length == 0)
return null;
const ord = Module.input_stdin_binary[0];
Module.input_stdin_binary = Module.input_stdin_binary.slice(1);
return ord;
},
```

I'm using this to calculate sha1 hash with busybox's sha1sum. I'm comparing providing file contents as stdin and calling sha1sum versus saving the contents to a temp file and calling sha1sum. The first is so much slower - probably for no good reason