Port to rustix, and add support for using Linux's renameat2

[sunfishcode]

This ports tempfile to use the rustix crate instead of making direct libc calls, which factors out an unsafe block, simplifies error handling, and simplifies string handling.

While here, it also implements a suggestion in a comment; on recent Linux it uses renameat2 with RENAME_NOREPLACE to implement persist in non-overwrite mode.

This does require incrementing the minimum required Rust version to 1.48. Please feel free to decline this PR if you don't wish to take on these new requirements.

[polarathene]

@Stebalien any concerns delaying review on this PR?

---

On a related note (due to the PR author), next month Rust 1.63 should have the new I/O Safety traits in std.

Until it's acceptable to raise the MSRV to >=1.63 to use those traits directly from std, in the meantime io-lifetimes could be adopted to cover these sections AFAIK?:

tempfile/src/file/mod.rs

Lines 968 to 988 in 716e58e

	#[cfg(unix)]
	impl<F> std::os::unix::io::AsRawFd for NamedTempFile<F>
	where
	F: std::os::unix::io::AsRawFd,
	{
	#[inline]
	fn as_raw_fd(&self) -> std::os::unix::io::RawFd {
	self.as_file().as_raw_fd()
	}
	}
	#[cfg(windows)]
	impl<F> std::os::windows::io::AsRawHandle for NamedTempFile<F>
	where
	F: std::os::windows::io::AsRawHandle,
	{
	#[inline]
	fn as_raw_handle(&self) -> std::os::windows::io::RawHandle {
	self.as_file().as_raw_handle()
	}
	}

tempfile/src/file/imp/windows.rs

Lines 49 to 64 in 716e58e

	pub fn reopen(file: &File, _path: &Path) -> io::Result<File> {
	let handle = file.as_raw_handle();
	unsafe {
	let handle = ReOpenFile(
	handle as HANDLE,
	FILE_GENERIC_READ | FILE_GENERIC_WRITE,
	FILE_SHARE_DELETE | FILE_SHARE_READ | FILE_SHARE_WRITE,
	0,
	);
	if handle == INVALID_HANDLE_VALUE {
	Err(io::Error::last_os_error())
	} else {
	Ok(FromRawHandle::from_raw_handle(handle as RawHandle))
	}
	}
	}

@sunfishcode may offer to assist with that as they've expressed interest in contributing this improvement to popular crates, but it probably helps if their contribution efforts aren't left to stagnate without feedback? 😅

[Stebalien]

Sorry, this fell off my radar. LGTM pending CI (which I'm trying to get to run now...).

[Stebalien]

Until it's acceptable to raise the MSRV to >=1.63 to use those traits directly from std, in the meantime sunfishcode/io-lifetimes#38 to cover these sections AFAIK?:

This seems reasonable to me. Just make sure to ping me when it's ready for review.