First Try: Let mmap allocator in Prof.Leis's ART & do a benchmark

[JigaoLuo]

Approach1:

/// Allocator
std::allocator<uint8_t> allocator;

---

Approach2:

/// Allocator
std::allocator<void> allocator;
using Allocator = std::allocator<void>;

/// Allocator for each node after rebinding
using Node4Allocator = typename Allocator::template rebind<Node4>;
using Node16Allocator = typename Allocator::template rebind<Node16>;
using Node48Allocator = typename Allocator::template rebind<Node48>;
using Node256Allocator = typename Allocator::template rebind<Node256>;