Improve mock_impl_runtime_apis! (#7370)

* Improve `mock_impl_runtime_apis!`

This adds a new attribute for functions being implemented in the
`mock_impl_runtime_apis!` macro, the `advanced` attribute. When this
attribute is given the user gets access to the `at` parameter and is
able to return a `Result`, instead of letting the macro generate this
stuff.

* Use the `at_param_name` directly

* Prevent clashing of `params`

substrate/primitives/core/src/lib.rs

@@ -196,6 +196,13 @@ pub enum NativeOrEncoded<R> {
 	Encoded(Vec<u8>)
 }
 
+#[cfg(feature = "std")]
+impl<R> From<R> for NativeOrEncoded<R> {
+	fn from(val: R) -> Self {
+		Self::Native(val)
+	}
+}
+
 #[cfg(feature = "std")]
 impl<R: codec::Encode> sp_std::fmt::Debug for NativeOrEncoded<R> {
 	fn fmt(&self, f: &mut sp_std::fmt::Formatter) -> sp_std::fmt::Result {