ralloc: Introduce new macros for defining C++ new/delete operators.

ralloc: Introduce new macros for defining C++ new/delete operators.

Most of our C++ classes define placement new and delete operators so we
can do convenient allocation via:

   thing *foo = new(mem_ctx) thing(...)

Currently, this is done via a lot of boilerplate.  By adding simple
macros to ralloc, we can condense this to a single line, making it
trivial to add this feature to a new class.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>