Add hidden __tag for union variables.

core/os.odin

@@ -1,3 +1,37 @@
 #load "os_windows.odin" when ODIN_OS == "windows";
-#load "os_x.odin" when ODIN_OS == "osx";
-#load "os_linux.odin" when ODIN_OS == "linux";
+#load "os_x.odin"       when ODIN_OS == "osx";
+#load "os_linux.odin"   when ODIN_OS == "linux";
+
+write_string :: proc(fd: Handle, str: string) -> (int, Errno) {
+	return write(fd, []byte(str));
+}
+
+read_entire_file :: proc(name: string) -> ([]byte, bool) {
+	fd, err := open(name, O_RDONLY, 0);
+	if err != ERROR_NONE {
+		return nil, false;
+	}
+	defer close(fd);
+
+	length: i64;
+	if length, err = file_size(fd); err != ERROR_NONE {
+		return nil, false;
+	}
+
+	if length == 0 {
+		return nil, true;
+	}
+
+	data := make([]byte, length);
+	if data == nil {
+		return nil, false;
+	}
+
+	bytes_read, read_err := read(fd, data);
+	if read_err != ERROR_NONE {
+		free(data);
+		return nil, false;
+	}
+	return data[0..<bytes_read], true;
+}
+