Mark C bindings as unsafe or don't make them public

[ammaraskar]

Hi there, we (Rust group @sslab-gatech) are scanning crates on crates.io for potential soundness bugs. We noticed that the bayer::ffi module contains functions intended to serve as C bindings:

https://github.com/wangds/libbayer/blob/master/src/ffi.rs

However, these should probably be marked as unsafe or not be exposed publicly in the crate, otherwise it allows users to potentially cause memory safety issues from safe Rust by passing wrong raw pointers to the ffi functions.