Silence useless and confusing extra output by tests
Fix invoking Singular interpreter functions with zero parameters
Fix GAP->Singular conversion of GMP ints for Singular debug builds
Handle orphan qrings in gapwrap() correctly

... although I now kind of doubt that they can ever occur in the first
place...
Minor cleanups
gapwrap: get rid of OBJ_COPY_OR_REF, we can simply always use CopyD
Improve gapwrap()

This fixes a problem where we’d keep incrementing the refcount of
singular rings each time they pass through gapwrap(). The code now tries
to be extremely conservative and *always* calls obj.CleanUp() before
returning.
Mark GAP wrappers for Singular strings as immutable.

Singular strings cannot be modified on the Singular interpreter level.
Hence some Singular (library) code might rely on that, and be confused
if Singular strings are changed after creation.

We can revise this revision in the future if we deem it absolutely
necessary, but for now, and after discussion with Hans, I prefer to go
with the conservative approach.