Mark Core struct as pub(crate) instead of [doc(hidden)]

[ammaraskar]

Hi there, we (Rust group @sslab-gatech) are scanning crates on crates.io for potential soundness bugs. We noticed that the Retro struct which allows for some unsafe operations is marked as #[doc(hidden)]:

https://github.com/koute/libretro-backend/blob/9248d749174bfb50641630c149d8529f1ba65a30/src/lib.rs#L208-L209

However, this still allows one to import this struct in and potentially use it to trigger unsafe operations from safe Rust code. Instead, maybe this struct should be pub(crate) to actually restrict it's usage?