serialization of more group attributes

[ThomasBreuer]

The idea is to implement serializations directly for the underlying GAP objects, instead of implementing them for the Oscar objects.

For boolean and numeric attributes, the two alternatives do not really differ, but for attributes whose values are themselves groups, for example center(g), we can simply serialize GAP.Globals.Center(GapObj(g)), together with its attributes, instead of first creating center(g) anew and then serializing its first component.

Another advantage is that this way, it will be easier to deal with "Oscar attributes" of groups that do not rely on GAP's attribute mechanism.

(The serialization of permutation groups gets changed with this approach; if we proceed like this then a fallback must be provided, in order to deal with the deserialization of files containing permutation groups that were serialized with the old method.)

Currently there is a problem with cyclic references, and the handling of attribute values that are GAP lists is not yet supported.

Here is an example for the cyclic reference problem:

julia> g = small_group(4, 1);

julia> path = mktempdir();  filenamev = joinpath(path, "v");

julia> save(filenamev, g)

julia> Oscar.reset_global_serializer_state();

julia> loadedv = load(filenamev);

julia> has_center(g)
false

julia> center(g);

julia> save(filenamev, g)

julia> Oscar.reset_global_serializer_state()
Dict{Base.UUID, Any}()

julia> loadedv = load(filenamev)
Internal error: encountered unexpected error during compilation of showerror:
StackOverflowError()