The commit implements a guarded allocator which can be used by STL classes
such as vectors, maps and so on. This allocator will keep track of current
and peak memory usage which then can be queried.
New code for allocator is only active when building Cycles with debug flag
(WITH_CYCLES_DEBUG) and doesn't distort regular builds too much.
Additionally now we're using own subclass of std::vector which allows us
to implement shrink_to_fit() method which would ensure capacity of the
vector is as big as it should be (without this making vector smaller will
still use all previous memory allocated).
