cleanup: Don't use GSlice anywhere
GSlice API is going to be deprecated [1] and marked as such for some years now, even if this change didn't happen yet, there's no point to keep GSlice usage in gjs, considering that we can semplificate it with calls to system allocator using g_new/g_malloc and g_free.
While changing this I've noticed that the sanitizer started complaining about leaks that we had for some time around but that GSlice usage was hiding for some reasons, as we were never unreffing trampolines in various situations. So fixing them and adding few cleanups.
The leaks fixed by commit d4ba600e are visible in this testlog, while commit dce3e759 fixes these leaks (same source though).
Edited by Marco Trevisan