syntax: make deriving have slightly less cryptic error messages.

This unfortunately changes an error like

    error: mismatched types: expected `&&NotClone` but found `&NotClone`

into

    error: type `NotClone` does not implement any method in scope named `clone`

src/etc/generate-deriving-span-tests.py

@@ -118,7 +118,7 @@ traits = {
 for (trait, supers, errs) in [('Rand', [], 1),
                               ('Clone', [], 1), ('DeepClone', ['Clone'], 1),
                               ('Eq', [], 2), ('Ord', [], 8),
-                              ('TotalEq', [], 2), ('TotalOrd', ['TotalEq'], 2)]:
+                              ('TotalEq', [], 1), ('TotalOrd', ['TotalEq'], 1)]:
     traits[trait] = (ALL, supers, errs)
 
 for (trait, (types, super_traits, error_count)) in traits.items():