polishing the new Julia to GAP conversion

[ThomasBreuer]

The following ideas arose in the discussion of #1029.

• Change the definition of recursive conversion: When converting to GAP, simply return a given GapObj. When converting to Julia, simply return a given non-GapObj. The differences to the current behaviour are that Julia subobjects of a GapObj will not get converted anymore, and that less copying happens (the results of the conversions will contain more subobjects that are identical to subobjects of the input). The new concept must be documented.
  We think that this new concept is easier to understand, and that deeply nested objects containing both GAP subobjects and Julia objects are not of practical interest.
• Shorten the syntax, in particular introduce a macro that simplifies the recursion handling.