Implementing AtomDict in C++

This is an attempt at implementing an atomdict equivalent to the atomlist. One issue remains for setdefault and leads to a segfault so this is not ready to merge. If you spot any mistake please let me know, as this is the first time I work with C extension I might be wrong in numerous places.
I plan also to implement a ContainerDict but I might do this in another PR once this one is merged.