Share conversion code between LibGAP.jl and JuliaInterface

[fingolfin]

Right now, many conversions are implemented twice: once in the GAP C kernel extension, see e.g. _ConvertedFromJulia_internal and _ConvertedToJulia_internal; and once in Julia, see from_gap and to_gap in LibGAP.jl/src/conversion.jl.

There are multiple problems with that:

• we have to duplicate a lot of work
• which conversions resp. types are supported can and will differ
• error checking differs
• one side may support other types than the other

Therefore, we should think about ways that allow us to implement all these conversions once for both sides. Whether this is done in C, GAP or Julia, or a mix of them does not matter that much to me, as long as it is efficient and available from both sides, and flexible enough for our purposes.