Revisit GAP.GAPFuncs

[fingolfin]

Right now, all GAP library level functions are automatically exported to Julia as GAP.GAPFuncs when the JuliaInterface package is loaded.

There are some issues with that, though: AddGapJuliaFuncs is only called once at the start, so any GAP functions added later on are not exported to Julia. This can be solved by calling the (currently undocumented) function AddGapJuliaFuncs. But I note that this takes ~700 milliseconds on my laptop, which isn't so nice (also in general, as it delays start up).

Perhaps GAP.GAPFuncs could be implemented in a fashion similar to what we now do for the Julia object on the GAP side, i.e., GAP.GAPFuncs.foo is looked up dynamically.

Alternatively, having a GAP.WrapGAPFunc function (in addition to or instead of GAP.GAPFuncs) and then writing e.g. MyFunc = GAP.WrapGAPFunc("MyFunc") in .jl files doesn't sounds so bad to me either.

[fingolfin]

This was resolved by PR #97