Improve and test support for custom functions

* NamesLocalVariablesFunction actually is a kernel operation, mark it as such
* fix a crash caused by infinite recursion when trying to display a function
  which is not a `T_FUNCTION`
* add tests for installing custom methods for NameFunction,
  NamesLocalVariablesFunction, and NumberArgumentsFunction

lib/function.g

@@ -243,7 +243,7 @@ DeclareOperationKernel( "NumberArgumentsFunction", [IS_OBJECT], NARG_FUNC );
 ##  </ManSection>
 ##  <#/GAPDoc>
 ##
-BIND_GLOBAL( "NamesLocalVariablesFunction", NAMS_FUNC );
+DeclareOperationKernel( "NamesLocalVariablesFunction", [IS_OBJECT], NAMS_FUNC );
 
 
 #############################################################################

lib/function.gi

@@ -63,6 +63,9 @@ end);
 InstallMethod(DisplayString, "for a function, using string stream", [IsFunction],
 function(fun)
     local  s, stream;
+    if TNUM_OBJ(fun) <> T_FUNCTION then
+      TryNextMethod();
+    fi;
     s := "";
     stream := OutputTextString(s, true);
     PrintTo(stream, fun);

tst/testinstall/callfunc.tst

@@ -29,12 +29,20 @@ gap> ForAll([0..100], x -> CallFuncListWrap(swallow, List([1..x], y -> [y]) ) =
 true
 
 # test overloading CallFuncList
-gap> fam := NewFamily("XYZsFamily");;
-gap> cat := NewCategory("IsXYZ",IsObject);;
+gap> fam := NewFamily("CustomFunctionFamily");;
+gap> cat := NewCategory("IsCustomFunction", IsFunction);;
 gap> type := NewType(fam, cat and IsPositionalObjectRep);;
 gap> result := fail;;
 gap> InstallMethod(CallFuncList,[cat,IsList],function(func,args) result:=args; return args; end);
-gap> o := Objectify(type,[]);;
+gap> InstallMethod(NameFunction, [cat], f -> f![1]);
+gap> InstallMethod(NamesLocalVariablesFunction, [cat], f -> ["arg"]);
+gap> InstallMethod(NumberArgumentsFunction, [cat], f -> -1);
+
+#
+gap> o := Objectify(type,["myName"]);
+function( arg... ) ... end
+gap> Display(o);
+<object>
 
 # test dispatch through interpreter / IntrFuncCallEnd
 gap> o();
@@ -109,7 +117,7 @@ gap> CallFuncList(o, [1,2,3,4,5,6,7]);
 [ 1, 2, 3, 4, 5, 6, 7 ]
 
 # test overloading CallFuncList with a procedure call
-gap> cat2 := NewCategory("IsXYZ2",IsObject);;
+gap> cat2 := NewCategory("IsCustomFunction2",IsFunction);;
 gap> type2 := NewType(fam, cat2 and IsPositionalObjectRep);;
 gap> InstallMethod(CallFuncList,[cat2,IsList],function(func,args) result:=args; end);
 gap> o2 := Objectify(type2,[]);;