ir: convert version fields to OFTypeMembers

The frontend gets a new OFVersionMember field type, which is lowered to
OFTypeMember when generating the IR (which is when the version is first known).
diff --git a/loxi_front_end/frontend_ir.py b/loxi_front_end/frontend_ir.py
index a927f94..af3f223 100644
--- a/loxi_front_end/frontend_ir.py
+++ b/loxi_front_end/frontend_ir.py
@@ -38,6 +38,7 @@
     'OFLengthMember',
     'OFFieldLengthMember',
     'OFPadMember',
+    'OFVersionMember',
     'OFEnum',
     'OFEnumEntry'
 ]
@@ -128,6 +129,16 @@
 OFPadMember = namedtuple('OFPadMember', ['length'])
 
 """
+Field with the version of an OpenFlow object
+
+@param name
+@param oftype C-like type string
+
+Example: hello.version
+"""
+OFVersionMember = namedtuple('OFVersionMember', ['name', 'oftype'])
+
+"""
 An OpenFlow enumeration
 
 All values are Python ints.