Make allocate take a template type

So we use the right alignment

benchmarks/bench_arena_allocator.cpp

@@ -16,7 +16,7 @@ int main() {
       // Arena allocator benchmark
       ArenaAllocator arena;
       bench.run("ArenaAllocator", [&] {
-        void *ptr = arena.allocate(alloc_size);
+        void *ptr = arena.allocate_raw(alloc_size);
         ankerl::nanobench::doNotOptimizeAway(ptr);
       });
     }