Output, to_ascii: Escape non-ascii chars with \xnn instead of \unnnn whenever possible

Previously ASCII-only mode was a bit wasteful:

```
$ uglifyjs --beautify ascii_only=true
alert("føø");
^D
alert("f\u00f8\u00f8");
```
With this patch the output is:

```
alert("f\xf8\xf8");
```