Remove libc dependencies

[Tacha-S]

• Replace libc crate with std::ffi
• Remove realloc and memcpy workaround https://github.com/ros2-rust/ros2_rust/issues/215#issuecomment-1279723455

[esteve]

@Tacha-S this is looking great, thanks! Let us know when it's ready for review

[Tacha-S]

I was developing with 1.64. But unfortunately 1.63(CI version) requires core::ffi instead of std::ffi.

https://doc.rust-lang.org/src/std/ffi/mod.rs.html#157

Which version should be support?

[nnmm]

I don't fully understand – the compiler error sounds like 1.63 has core::ffi, but the release notes and core::ffi docs look like that module was introduced with 1.64? Maybe these types were available but experimental in 1.63.

Either way, I think it would be nice to support 1.63 if it's not too ugly. Maybe that requires using std::os::raw after all.

[Tacha-S]

core::ffi was unstable before 1.64.0.