List slice-self-assignment is currently buggy

Not sure how hard a requirement it is to match CPython's behavior,
but just assert out for now rather than doing something different.

Also add an assert to copySlice, which could have similar issues.

src/runtime/list.cpp

@@ -210,6 +210,8 @@ extern "C" Box* listSetitemSlice(BoxedList* self, BoxedSlice* slice, Box* v) {
     ASSERT(v->cls == list_cls, "unsupported %s", getTypeName(v)->c_str());
     BoxedList* lv = static_cast<BoxedList*>(v);
 
+    RELEASE_ASSERT(self->elts != lv->elts, "Slice self-assignment currently unsupported");
+
     int delts = lv->size - (stop - start);
     int remaining_elts = self->size - stop;
     self->ensure(delts);

src/runtime/util.h

@@ -24,6 +24,7 @@ class BoxedSlice;
 void parseSlice(BoxedSlice* slice, int size, i64* out_start, i64* out_stop, i64* out_end, i64* out_length = nullptr);
 
 template <typename T> void copySlice(T* __restrict__ dst, const T* __restrict__ src, i64 start, i64 step, i64 length) {
+    assert(dst != src);
     if (step == 1) {
         memcpy(dst, &src[start], length * sizeof(T));
     } else {