[buffer] Implement buffer deserialization for format=text

Using a ragel machine.
diff --git a/src/hb-buffer.h b/src/hb-buffer.h
index 56f04d3..55a4045 100644
--- a/src/hb-buffer.h
+++ b/src/hb-buffer.h
@@ -313,7 +313,7 @@
 hb_buffer_deserialize_glyphs (hb_buffer_t *buffer,
 			      const char *buf,
 			      int buf_len, /* -1 means nul-terminated */
-			      unsigned int *buf_consumed, /* May be NULL */
+			      const char **end_ptr, /* May be NULL */
 			      hb_font_t *font, /* May be NULL */
 			      hb_buffer_serialize_format_t format);