Defining dedicated macro to grab a relation's persistence

Hi all,

After looking at a patch of this commit fest using
rd_rel->relpersistence, I got a look at how many times this expression
was being used directly in the backend code and wondered if it would
not be useful to add a dedicated macro in rel.h to get the persistence
of a relation like in the patch attached. (Note: it is actually used
39 times).
Thoughts?
--
Michael