Allow use of CGP proc macros without explicit import of prelude

[soareschen]

Currently, the CGP pro macros are unhygenic, and requires import of cgp::prelude::* for the expanded syntax to work. Unfortunately, there is no equivalent of $crate in the proc macro world to help us bind CGP constructs with proper module path.

We would need to figure out how to properly scope CGP constructs inside the macro expansion, so that the macros could work even if users don't import the prelude.

[gkgoat1]

You can probably use ::cgp as a default with an attribute/attribute parameter to change the crate path in nested macro invocations