Add GAP_CallFunc[0-3]Args

[fingolfin]

This are purely for convenience, to avoid the need to create a C array of arguments.

@embray might be interested

[embray]

This could be useful in a few places where GAP_CallFunc is used directly, though for calling an arbitrary GAP function the more general array-based interface is the most useful. In the current gappy I found only 5 instances of GAP_CallFuncArray all with either 1 or 2 arguments, so this would be a minor convenience there.