[mycpp/runtime] Make MarkSweepHeap::Reallocate() NotImplemented (#1412)

Some discussion here as to why: https://oilshell.zulipchat.com/#narrow/stream/121539-oil-dev/topic/Simple.20Mark.20and.20Sweep.20Design.20for.20a.20Shell/near/310129879

mycpp/marksweep_heap.cc

@@ -52,7 +52,7 @@ void* MarkSweepHeap::Allocate(size_t num_bytes) {
 }
 
 void* MarkSweepHeap::Reallocate(void* p, size_t num_bytes) {
-  return realloc(p, num_bytes);
+  NotImplemented();
 }
 
 #elif defined(BUMP_LEAK)