Add pretty printer for platform_x86_64::arg_pass_class enum.
Bug fix in rule D.2 for argument passing

Need to push another arg_passing_class for every argument, rather than
for every 8-byte.
Bug fix: 8-byte & SSE rule: squash tuple list if passed by memory.
Bug fix: 8-byte & SSE rule: squash tuple list if passed by memory.
Add static_assert to APT_iter to check that classification is consistent across 8-bytes
Bug fix in argument passing for structures.

Bad book-keeping of structure element types