add of_str64_t string type

We need a 64-byte string for the debug_counter extension. Since adding a new
string type is so much work, the new type is named generically so that any
extension can use it.
diff --git a/loxi_ir/ir_offset.py b/loxi_ir/ir_offset.py
index e705fcc..2e6d004 100644
--- a/loxi_ir/ir_offset.py
+++ b/loxi_ir/ir_offset.py
@@ -95,6 +95,7 @@
     of_table_name_t = (ofp_constants["OF_MAX_TABLE_NAME_LEN"], True),
     of_desc_str_t = (ofp_constants["OF_DESC_STR_LEN"], True),
     of_serial_num_t = (ofp_constants["OF_SERIAL_NUM_LEN"], True),
+    of_str64_t = (64, True),
     of_match_v1_t = (40, True),
     of_match_v2_t = (88, True),
     of_match_v3_t = (8, False),