Minimize memory usage by having an arena per thread rather than an arena per file

src/array.cpp

@@ -89,7 +89,9 @@ template <typename T>
 void slice_init(Slice<T> *s, gbAllocator const &allocator, isize count) {
 	GB_ASSERT(count >= 0);
 	s->data = gb_alloc_array(allocator, T, count);
-	GB_ASSERT(s->data != nullptr);
+	if (count > 0) {
+		GB_ASSERT(s->data != nullptr);
+	}
 	s->count = count;
 }