`stack-graphs` C API exports are safe `extern "C"` functions

stackgraphs::c is a public module. From 0.0.3 through the current crates.io release 0.14.1, its pointer-taking entry points are pub extern "C" fn rather than unsafe fn. Safe Rust can call them. sgstackgraphfree frees the pointer with Box::fromraw. sgstackgraphfreestd::ptr::nullmut is immediate...