Use std::os::raw types instead of libc crate

[nnmm]

It seems that the libc crate is only really useful in #[no_std] environments. https://stackoverflow.com/questions/44436515/should-i-use-libcc-char-or-stdosrawc-char

We should see if we can replace all uses of libc with std::os::raw.

[Tacha-S]

I found libc crate use of the following items within this project.

• c_void
  • It is used in *mut c_void, in which case both libc and std::os::raw are equivalent to C's void*.
• c_char(Both libc and std are used.)
  • Exist in std::os::raw.
• c_ushort
  • Exist in std::os::raw.
• uintptr_t
  • Not implemented in std::os::raw, but may be replaced by usize.
• realloc
  • Not implemented in std::os::raw. Compatibility with C is not specified, but std::alloc::realloc is the implementation in Rust.
• memcpy
  • Not implemented in std::os::raw, but copy_nonoverlapping is semantically equivalent to C memcopy.
• size_t
  • Not implemented in std::os::raw. However, since --size_t_is_usize is enabled in bindgen, it can be replaced by usize.(ref)

ros2_rust/rclrs/build.rs

Line 37 in 0c26297

.size_t_is_usize(true)

I consider c_void, c_char, c_ushort, uintptr_t to be fine, what about realloc, memcpy, size_t?

[nnmm]

I agree about replacing the integer aliases like you say. We could also use std::ffi instead of std::os::raw, I actually think that's a bit nicer.

For the allocation functions, they are currently only used in one place iirc: In sequence.rs, as a workaround for some functions that were missing in older distros. I had already backported them in ros2/rosidl#667 but they hadn't landed in a Foxy release yet when I wrote sequence.rs. However, I just checked and they've now been released, so there is no need for the workaround anymore, and therefore for realloc and memcpy.

[Tacha-S]

Thank you for the background on realloc and memcpy.

I will create a draft with the replacement with std::ffi and the elimination of realloc and memcpy.

[nnmm]

Closed in #284