Add basic file-system functionality

std.fs.list_dir will list the files in a directory, std.fs.file_is_dir
will, given a pathname, determine whether it is a directory or not.

src/lib/win32_fs.rs

@@ -0,0 +1,10 @@
+native "rust" mod rustrt {
+  fn rust_list_files(str path) -> vec[str];
+  fn rust_file_is_dir(str path) -> int;
+}
+
+impure fn list_dir(str path) -> vec[str] {
+  ret rustrt.rust_list_files(path+"*");
+}
+
+const char path_sep = '\\';