Segment 0, byte 0 is a request code.  Currently defined 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); however,
offsets encoded in the message are relative to the beginning of the
segment, not the payload.  The reserved bytes do not need to be
included in the message if the payload is empty.

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

The payload begins with the index of the interface being used,
followed by the index of the method.  Both are 32-bit integers.

After that, the payload contains the method's arguments as if they
were a struct.

Non-inline structs and arrays (hereafter referred to as data chunks)
which contain object ids and/or references to other data chunks are
stored at arbitrary locations in segment 0, and are referenced as an
offset into the segment.

Data chunks which do not contain object id or references to other
data chunks (i.e. they are pure data, and do not need to be altered
for marshalling) are stored at arbitrary locations in arbitrary
segments.  References to these pure data chunks are encoded as a
pointer-sized integer, with the upper N bits holding the segment and
the remaining bits holding the offset into that segment.  The value
of N is architecture-defined, but should generally be 6 (this may
change once enough of the system is in place to do some benchmarks)
for 32-bit architectures and 32 for 64-bit architectures.

Object references are encoded as pointer-sized offsets into the
objlist array.

Virtual structs are prefaced with a header, which consists of
the following, in order:
1. 128-bit GUID of the most derived vstruct type
2. 160-bit SHA-1 of the vstruct type info
3. 32-bit objlist-offset of an introspection object
4. 32-bit length of vstruct, including header, and all referenced
data chunks, which must be contiguously appended to the vstruct body
5. 32-bit reserved field

The requirement for all referenced data chunks to be contiguous with
the vstruct means that scatter-gather and zero-copy cannot be done;
however, it prevents the loss of derived-type data when only a base
type is known at unmarshalling time (the derived-type data could
later be accessed through introspection, or be passed to another
process that does know the derived type).  For this reason, vstructs
should generally not be used to pass large chunks of data.

The introspection object reference may be NULL; in this case, the
SHA-1 field is ignored.

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.

Segment 1 consists of a list of GUIDs of interfaces that the object
supports.