Public API of bp7 exposes UB to safe code

[dtolnay]

For example, the following safe program leads to bp7 performing Undefined Behavior:

// [dependencies]
// bp7 = "0.9.2"

fn main() {
    bp7::ffi::bundle_payload(0xdeadbeefusize as _);
}

Segmentation fault (core dumped)

In general it is unsound for a safe API to result in UB.

[gh0st42]

Thank you for pointing this out!

I experimented with different alternatives but would would you suggest as the cleanest solution?

As a rust crate the whole FFI module is not needed and should not be used.
To create the headers using cbindgen the mentioned function must be made public.
Thus, it appears also in the rust crate.

Gating it behind a feature flag makes it a bit harder to accidentally misuse it but in the end the UB could still end up in rust code.

Adding unsafe to all functions pleases clippy but should not extern "C" already imply that it is unsafe or contains potential UB?