* 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`