Display warning on stderr, not stdout.
Display warning on stderr, not stdout.

Right now, a part of the warning (undefined variable: foo, is exported by: ..)
is displayed to stdout. This patch makes the entire message show up on stderr,
not split up between the two output streams.

holmescn/chibi-scheme,holmescn/chibi-scheme,holmescn/chibi-scheme,holmescn/chibi-scheme,holmescn/chibi-scheme