Add a mechanism to easily "pin" references to Julia objects in our C code

[fingolfin]

One way would be to add them to the GAP module state, as those automatically get marked. Or we could add these to "our" Julia module. Or we could set a custom ExtraMarkFuncBags in GAP to a function marking the relevant variables.

[fingolfin]

I don't think we need to keep this open as an issue, we'll just deal with it as/if it comes up.