Replace `Julia.GAP` by a cleaner solution?

[fingolfin]

Right now we use a hack to make sure Julia.GAP always refers to the GAP julia module, even if it was not loaded into Julia's Main module.

This requires some hacky code. It also means that Main.GAP in Julia and Julia.GAP in GAP can reference different objects (mind you, that's not a major concern as it is easy to avoid for a user, but it's still annoying).

So perhaps we can provide an additional alternative means to access it (and then phase out use of Julia.GAP over time. E.g. we could add a global variable GAP_jl which contains the GAP module. Suggestions for better names welcome. (I would not call it GAP, though, as I believe this would be very confusing for GAP users).

If we went with GAP_jl, then for symmetry in Oscar we could also provide Oscar_jl as an alias for the Oscar global variable.