Let the allocator determine if it should be inherited by new thread.

[xzores]

I am asking for a feature for the allocators which could be a boolean or maybe even a proc, which lets the allocator define what allocator a new thread should inherit.

I find myself having problem with this when i use the tracking allocator or try and measure memory usage across threads, and I think that it is actually the context.allocator which should determine what allocator the thread should use (of course this should be able to be overruled by the user).