Providing the POSIX layer for a `dart:io` reimplementation

I'm on the Dart team at Google and just started working on reimplementing the `dart:io` file system API in Dart (see [`package:io_file`](https://github.com/dart-lang/labs/tree/main/pkgs/io_file)).

Currently, I'm using `package:stdlibc.dart` for the POSIX file system implement (e.g. [`rename`](https://github.com/dart-lang/labs/blob/6215560d0a7f9ab455e7232bc3880ae0a84f0328/pkgs/io_file/lib/src/posix_file_system.dart#L34)).

Before going too far, I just wanted to make sure that you are OK with me depending on `package:stdlibc.dart` and that you'd be open to reviewing PRs that add the functionality that I need (like `rename` ;-)). For some things, I'd like to add support for getting the raw results of the system call (for example, getting `st_atim`/`st_ctim`/`st_mtim` as `int` rather than Dart `DateTime` (because `DateTime` only has millisecond precision)).

Let me know if you see any issues and happy to talk about this in a different forum! And thanks for the great package!