Output to ASCII

[ExpHP]

Currently this can output things like

{
    "β": 1
}

where it will use UTF-8 encoding on Linux and... whatever Windows feels like using on Windows. However, the encoding is not part of the JSON spec (it is defined on code points; not even scalar values!), and so I've had issues with other JSON parsers in the past when using files like this. This should ideally be escaped to "\u03b2".