Improve Rust implementation

- Move mmap_file implementation into Rust
- Introduce Utf8CStr as the better c-string type to use

native/src/base/lib.rs

@@ -38,5 +38,9 @@ pub mod ffi {
         fn xpipe2(fds: &mut [i32; 2], flags: i32) -> i32;
         #[rust_name = "fd_path_for_cxx"]
         fn fd_path(fd: i32, buf: &mut [u8]) -> isize;
+        #[rust_name = "map_file_for_cxx"]
+        fn map_file(path: &[u8], rw: bool) -> &'static mut [u8];
+        #[rust_name = "map_fd_for_cxx"]
+        fn map_fd(fd: i32, sz: usize, rw: bool) -> &'static mut [u8];
     }
 }