syntax: rename TypeMethod to MethodSig and use it in MethDecl.

src/test/compile-fail/impl-duplicate-methods.rs

@@ -11,7 +11,7 @@
 struct Foo;
 impl Foo {
     fn orange(&self){}
-    fn orange(&self){}   //~ ERROR error: duplicate method in trait impl
+    fn orange(&self){}   //~ ERROR error: duplicate method
 }
 
 fn main() {}

src/test/compile-fail/issue-4265.rs

@@ -17,7 +17,7 @@ impl Foo {
         Foo { baz: 0 }.bar();
     }
 
-    fn bar() { //~ ERROR duplicate method in trait impl
+    fn bar() { //~ ERROR duplicate method
     }
 }