Add a new function to pgx-pg-sys, `create::submodules::postgres_function_guard()` and teach the `#[pg_guard]` macro to use that function instead of duplicating all the sigsetjmp code for every function.

…writer` from build.rs.

Also eliminate a bit of cloning from build.rs as well.

This drastically improves pgx-pg-sys build times.

On my machine, a clean build of the crate goes from ~36s to ~18s.  And subsequent rebuilds (where build.rs doesn't run) goes from ~8s to ~2.4s

…tions, which decreases subsequent pgx-pg-sys rebuilds (where build.rs needs to run) by ~50%.