Ensure allocations happen through custom allocators

Ensure allocations happen through custom allocators

Document instance_private_data and device_private_data.

Add additional utility containers (util::unordered_set,
util::unordered_map) that ensure that the storage containers
we use allow the use of the Vulkan allocation callbacks.
Additionally, ensure that these allocations don't throw in
case host runs out of memory but rather return the appropriate
Vulkan error code.
Also keep a copy the allocators so they can be used in other layer
functionality to facilitate following the Vulkan specification around
memory allocation.

Add additional utility util::optional that currently makes it easier
to handle errors in the containers described above.

Fix some small issues with the vkCreateInstance/vkCreateDevice
handling and ensure that vkDestroyDevice is called in the layer
handler.

Change-Id: Ic9d8ece405c82e743a2c016cc3dabf984cf77fc1
Signed-off-by: Normunds Rieksts <normunds.rieksts@arm.com>