support default impl for specialization

this commit implements the first step of the `default impl` feature:
all items in a `default impl` are (implicitly) `default` and hence
specializable.
In order to test this feature I've copied all the tests provided for the
`default` method implementation (in run-pass/specialization and
compile-fail/specialization directories) and moved the `default` keyword
from the item to the impl.
See referenced issue for further info

src/libsyntax_ext/deriving/generic/mod.rs

@@ -658,6 +658,7 @@ impl<'a> TraitDef<'a> {
                 a,
                 ast::ItemKind::Impl(unsafety,
                                     ast::ImplPolarity::Positive,
+                                    ast::Defaultness::Final,
                                     trait_generics,
                                     opt_trait_ref,
                                     self_type,