girnode.h: Store the total size and alignment for
GIrNodeStruct/Boxed/Union.
giroffset.c: New file implementing computation of structure
field offsets.
girnode.c: Compute structure field offsets before writing types
into the typelib.
docs/typelib-format.txt: Document that a field offset of 0xFFFF
means "unknown". Also fix description of the discriminator_offset
field for unions.
svn path=/trunk/; revision=876
