pgrx-macros

Procedural macros for pgrx.

Provides:

- #[pg_extern]
- #[pg_guard]
- #[pg_test]
- #[derive(PostgresType)]
- #[derive(PostgresEnum)]
- #[derive(PostgresGucEnum)]

Using pgrx as a dependency necessitates that pgrx-macros also be a dependency