debug is relied upon by a great deal of other units so it's very
difficult to include other popular headers like format.hpp without
triggering cyclic dependencies.
ipp files weren't a great way of keeping things clean, and IDEs have a
little trouble dealing with the split configuration. this simplifies
debugging a great deal.
GCC produces ODR error when including paths of the form './foo' and
'foo' in the same binary. Rather than managing duplication we just
universally pick the absolute form over the relative form.
