Provide extended allocators interface (e.g. grow/realloc) like Boost.Container #116

Boost.Container allocators support extra interfaces than the standard allocator requirements.

Some of these functions are useful even to C libraries that used to enjoy realloc() like features. So this extra interface is useful even if we're not planning to use it with standard containers.

It'd be nice if Boost.Interprocess could provide a few of these features for its allocators as managed_shared_memory has plenty of features not found in Boost.Container allocators and we cannot use Boost.Container extended allocators to manage preallocated memory regions.