segment 0, byte 0 is a request code.  Codes are:
 0: Invoke Method
 1: Get Interface List
 2: Return Interface List

Bytes 1-3 (32-bit) and 1-7 (64-bit) are reserved, and must be zero.
All offsets described in the rest of this file are relative to the
first non-reserved byte (a.k.a. the message payload).  The reserved
bytes do not need to be included in the message if the payload is
empty.

Op0: Invoke Method
------------------

The segment 0 payload contains the method's arguments as if they were
a struct, followed by a depth-first traversal of any referenced
structs, excluding references which could potentially lead to loops
(i.e. references to types that exist in a parent node in the
traversal, or their supertypes) or aliases (i.e. references to types
that exist elsewhere in the traversal, or their supertypes).  Where
loops or aliases are possible, the reference is encoded as a
segment/offset pair (a 32-bit word, segment is upper 6 bits), and the
data may be placed wherever the sender wishes.  Non-inline arrays are
also encoded with segment/offset pairs.  Object references are
encoded as 16-bit offsets into the objlist array.

Op1: Get Interface List
-----------------------

Return a "Return Interface List" message corresponding to the object
in objlist[0].  For async requests, objlist[1] is the destination for
the return message.

Op2: Return Interface List
--------------------------

For async requests, objlist[1] is the object whose interfaces were
requested.

The payload consists of a list of GUIDs of interfaces that the object
supports.