More trait infrastructure

- make it possible to get parent trait from method
 - add 'obligation' machinery for checking that a type implements a
   trait (and inferring facts about type variables from that)
 - handle type parameters of traits (to a certain degree)
 - improve the hacky implements check to cover enough cases to exercise the
   handling of traits with type parameters
 - basic canonicalization (will probably also be done by Chalk)

crates/ra_hir/src/impl_block.rs

@@ -84,7 +84,8 @@ impl ImplBlock {
     }
 
     pub fn target_trait_ref(&self, db: &impl HirDatabase) -> Option<TraitRef> {
-        TraitRef::from_hir(db, &self.resolver(db), &self.target_trait(db)?)
+        let target_ty = self.target_ty(db);
+        TraitRef::from_hir(db, &self.resolver(db), &self.target_trait(db)?, Some(target_ty))
     }
 
     pub fn items(&self, db: &impl DefDatabase) -> Vec<ImplItem> {