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