Replace remaining uses of gap_to_julia

[fingolfin]

... by conversions to specific Julia types. One step towards this is done in PR #4624, but other uses are left. Moreover, there are uses in Hecke.jl which also need to be dealt with.

The only thing that needs to be kept for now are the parts which define additional methods for gap_to_julia; those can be removed once we've really removed all gap_to_julia calls. That includes calls inside GAP.jl, i.e., we first need to complete the refactoring of the GAP->Julia conversion there.