regen/embed.pl: Refactor ARGS_ASSERT generation

This is in preparation for future enhancements.

This commit combines two arrays into one, placing them in their final
form as we go along.  Previously the two arrays had to be merged later,
with a grep to look for information that had been lost because of the
two-array scheme.

The order of some assertions in the generated proto.h are changed.  But
this makes sure that we continue to assert that an argument is present
before dereferencing it.

Author: khwilliamson