Remove the extra backtrace stuff.

With the recent changes to RustBox, main looks extremely straightforward
now, while still being totally safe.  The previous solution was not
quite correct since it would do things like duplicate error messages if
you redirected stderr, and only captured stderr output on panic!.  The
new solution lets you tell RustBox to automatically redirect stderr to
an internal pipe, which will get printed at the end of the program.
This allows us to avoid hooking into the panic system explicitly and is
also much easier for users to use.  It also lets us get rid of the final
unsafe block (again).