Support u128/i128 in runtime interface (#4703)

* Support `u128`/`i128` in runtime interface

This implements support for `u128`/`i128` as parameters/return value in
runtime interfaces. As we can not pass them as identity, as for the
other primitives types, we pass them as an pointer to an `[u8; 16]` array.

* Remove some unsafe code usage

substrate/primitives/runtime-interface/test/src/lib.rs

@@ -108,3 +108,8 @@ fn test_invalid_utf8_data_should_return_an_error() {
 fn test_overwrite_native_function_implementation() {
 	call_wasm_method::<HostFunctions>("test_overwrite_native_function_implementation");
 }
+
+#[test]
+fn test_u128_i128_as_parameter_and_return_value() {
+	call_wasm_method::<HostFunctions>("test_u128_i128_as_parameter_and_return_value");
+}