Use Unicode private-use characters for roundtripping

Use Unicode private-use characters for roundtripping

This replaces the previous scheme (which used lone surrogates). The reason is that
there is Haskell software in the wild (i.e. the text package) that chokes on Char values
that do not represent Unicode characters.

This new approach will not work correctly if the reserved private-use characters are
actually encountered in the input, but we expect this to be rare.