Test FFI safety

[dtolnay]

Given the name of the crate I would expect to have something to verify that the generated data structure is considered FFI-safe by rustc. I've seen rustc periodically introduce regressions in what is considered to be FFI-safe.

Something like:

#[deny(improper_ctypes, warnings)]
extern "C" {
    pub fn f(_: *const leveldb_t);
}

[nvzqz]

I tried making a macro for this (nvzqz/static-assertions#31), but #[deny] and #[forbid] do not pass through macros (rust-lang/rust#67720).

[skade]

Implemented in: Thanks for the suggestion!

533bfa6#diff-b1a35a68f14e696205874893c07fd24fdb88882b47c23cc0e0c80a30c7d53759