Add the latest test cases to Makefile
Add field_modifier() test case.
Implement testing for field_width()
Rework the %-esc parsing as a consequence of shortcomings exposed by
work on testing
First pass at tests for format_code().
Allow pkg_printf() functions to take struct 'pkg_foo *' subtypes
at the top level, as well as in pkg iterators.
Reorder pp_fmt_t so that multi-character codes come before single
characters.  This allows multi-character codes to be used at the top
level, and not just in format trailers.

Corresponding changes to tests, etc.
Add tests for format_trailer(), and apply consequential fixes to the
code.