This required adding a `var` symbol type. For now, it holds just the
storage class, but it might be good for the initializer, too.
Also, clean up some pointer/reference inconsistencies.
It wasn't quite as nasty as I thought, but splitting it up helps add a
little self documentation and, possibly more importantly, simplify
navigation through the type checks and argument setup.
I've long felt build_function_call was getting a bit big, and expr.c
especially so. This should make it easier to rewrite build_function_call
for dealing with target-specific code. As a bonus, the int through ...
warning is already cleaned up.
