testsuite: add tests for derived Eq, TotalEq, Ord, TotalOrd.
libsyntax: short-circuit on non-matching variants in deriving code.

Allow a deriving instance using the generic code to short-circuit for
any non-matching enum variants (grouping them all into a _ match),
reducing the number of arms required. Use this to speed up the Eq &
TotalEq implementations.