std.heap.ArenaAllocator should attempt to resize before allocating a new node

[andrewrk]

Currently ArenaAllocator does this:

            const cur_buf = cur_node.data[@sizeOf(BufNode)..];
            const addr = @ptrToInt(cur_buf.ptr) + self.end_index;
            const adjusted_addr = mem.alignForward(addr, alignment);
            const adjusted_index = self.end_index + (adjusted_addr - addr);
            const new_end_index = adjusted_index + n;
            if (new_end_index > cur_buf.len) {
                cur_node = try self.createNode(cur_buf.len, n + alignment);
                continue;
            }

After #4431 is implemented, this can be improved to attempt a resize before creating a new allocation. This will make it OK to use an ArenaAllocator as the backing allocator of an ArenaAllocator when it is the newest allocation.