libcore: Add stat methods to paths

Should we return the posix types, or should we just use i64s?
Should we expose platform specific stat fields?

src/test/run-pass/stat.rs

@@ -0,0 +1,24 @@
+extern mod std;
+use io::WriterUtil;
+use std::tempfile;
+
+fn main() {
+    let dir = option::unwrap(tempfile::mkdtemp(&Path("."), ""));
+    let path = dir.with_filename("file");
+
+    {
+        match io::file_writer(&path, [io::Create, io::Truncate]) {
+            Err(e) => fail e,
+            Ok(f) => {
+                for uint::range(0, 1000) |_i| {
+                    f.write_u8(0);
+                }
+            }
+        }
+    }
+
+    assert path.exists();
+    assert path.get_size() == Some(1000);
+
+    os::remove_dir(&dir);
+}