* sp-api: Support expanding the macro code

This pr introduces the `expander` crate to expand the generated source code into a file. This gives
better error reporting when trying to fix issues in the macro itself as Rustc will point to the line
in this file. The feature can be enabled by setting `SP_API_EXPAND=1` at compile time.

Besides that the generated code is changed to fix warnings in the exanped version.

* Fixes