cffi: we should probably not include capi stuff in the standard crate

[cyphar]

We set panic = "abort" unconditionally for our builds, which is not good for a general-purpose crate. And the capi code should probably not be included at all for Rust users (unless we decide to have libpathrs-sys for license reasons -- see #50 -- but you still wouldn't want to use it for standard uses).

cargo-c is meant to make this easier, so we can probably use that.