Add some magic to signed statements and approval votes (#2585)

* add a magic number to backing statements encoded

* fix fallout in statement table

* fix some fallout in backing

* add magic to approval votes

* remove last references to Candidate variant

* update size-hint

polkadot/statement-table/src/lib.rs

@@ -53,7 +53,7 @@ pub mod v1 {
 			match *s {
 				generic::Statement::Valid(s) => PrimitiveStatement::Valid(s),
 				generic::Statement::Invalid(s) => PrimitiveStatement::Invalid(s),
-				generic::Statement::Candidate(ref s) => PrimitiveStatement::Candidate(s.hash()),
+				generic::Statement::Seconded(ref s) => PrimitiveStatement::Seconded(s.hash()),
 			}
 		}
 	}