kernel: make OnLeftInverse use LQUO

This ensures that the mutability of the inputs correctly transfers to
the results. In addition, we now benefit from kernel functions
handling LQUO for e.g. permutations.

src/listfunc.c

@@ -1369,8 +1369,7 @@ Obj             FuncOnLeftInverse (
     Obj                 point,
     Obj                 elm )
 {
-    elm = INV(elm);
-    return PROD( elm, point );
+    return LQUO(elm, point);
 }
 
 /****************************************************************************

tst/testbugfix/2018-10-08-OnLeftInverse.tst

@@ -0,0 +1,26 @@
+# Ensure OnLeftInverse preserves the mutability of its
+# input arguments.
+gap> m:=IdentityMat(2);
+[ [ 1, 0 ], [ 0, 1 ] ]
+gap> IsMutable(m);
+true
+gap> IsMutable(m/m);
+true
+gap> IsMutable(LQUO(m,m));
+true
+gap> IsMutable(OnRight(m,m));
+true
+gap> IsMutable(OnLeftInverse(m,m));
+true
+
+#
+gap> MakeImmutable(m);
+[ [ 1, 0 ], [ 0, 1 ] ]
+gap> IsMutable(m/m);
+false
+gap> IsMutable(LQUO(m,m));
+false
+gap> IsMutable(OnRight(m,m));
+false
+gap> IsMutable(OnLeftInverse(m,m));
+false