remove the redundant copy when move item to the front of the lru list in lru_cache.hpp

[faker2048]

std::list<T,Allocator>::splice is the helper for to move list element in constant complexity.

Complexity
1-2) Constant.
3) Constant if other refers to the same object as *this, otherwise linear in std::distance(first, last).

Ref: https://en.cppreference.com/w/cpp/container/list/splice