Fix some coding issues in BRIN

Reported by David Rowley: variadic macros are a problem.  Get rid of
them using a trick suggested by Tom Lane: add extra parentheses where
needed.  In the future we might decide we don't need the calls at all
and remove them, but it seems appropriate to keep them while this code
is still new.

Also from David Rowley: brininsert() was trying to use a variable before
initializing it.  Fix by moving the brin_form_tuple call (which
initializes the variable) to within the locked section.

Reported by Peter Eisentraut: can't use "new" as a struct member name,
because C++ compilers will choke on it, as reported by cpluspluscheck.