Remove `scid.core.memory`, use new allocators package in Phobos instead

[kyllingstad]

...as soon as it actually gets included in Phobos, that is.

[Laeeth]

you can do what EMSI did with their containers and include a subrepo.

https://github.com/economicmodeling/containers
https://github.com/Hackerpilot/experimental_allocator