improve storage build type checking (#1914)

* force build closure in decl_storage! to return explicit type
* fix when type was different

Note: it breaks API but it is easy to upgrade: just do the conversion from encode type to the final type yourself in the build closure:

```rust
Encode::using_encoded(&value, |mut value| Decode::decode(&mut v)).unwrap();
```

substrate/node/runtime/src/lib.rs

@@ -61,7 +61,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	impl_name: create_runtime_str!("substrate-node"),
 	authoring_version: 10,
 	spec_version: 31,
-	impl_version: 31,
+	impl_version: 32,
 	apis: RUNTIME_API_VERSIONS,
 };