String buffers, part 2d: basic string buffer methods.

Sponsored by fmad.io.

src/lj_obj.h

@@ -332,6 +332,7 @@ enum {
   UDTYPE_USERDATA,	/* Regular userdata. */
   UDTYPE_IO_FILE,	/* I/O library FILE. */
   UDTYPE_FFI_CLIB,	/* FFI C library namespace. */
+  UDTYPE_BUFFER,	/* String buffer. */
   UDTYPE__MAX
 };