Add C++20 Supports

[changkun]

需要支持以下内容：

• 语言级 bug 与修订

• 类型系统改进 Type Punning
• 弃用和删除的功能
  • register
  • std::auto_ptr --> std::unique_ptr
  • std::random_shuffle
  • std::bind1st, std::bind2nd, ...

• 语言增强

• 结构化绑定 Structured bindings
• 新的控制结构 constexpr if
• 编译时条件语句 Compile-time conditional statements
• 新的聚合规则
• 强制性 RVO 和复制 elision, Guaranteed copy elision
• Unary static_assert
• 嵌套命名空间 Nested namespace deinitions
• inline 变量 inline variables
• constexpr 改进, constexpr lambda

• 模板增强

• 折叠表达式 Fold expressions
• 类模板参数推导 Class template deduction
• variadic templates fold
• auto non-type template parameters
• Preprocessor predicate for header testing

• 容器

• std::string_view, std::byte 加入 container 容器一章
• std::any, std::variant, std::optional 加入 container 容器一章
• 容器改进

• 线程

• thread 并行 STL 算法
• std::shared_mutex, atomic<T>::is_always_lockfree, scoped_lock<Mutexes...>

• 文件系统

• [文件系统库, 专门用一章来讲解](http://en.cppreference.com/w/cpp/filesystem, http://www.bfilipek.com/2017/08/cpp17-details-filesystem.html)

• 数学库

• 对机器学习的影响 Math special functions

• 其他特性

• Polymorphic allocators and memory resources
• Aligned new
• Improved insertion and splicing for associative constrainers
• Boolean logic metafunctions
  ...

• 展望 C++20

• concept
• module
• coroutine
• Ranges

[kassane]

Suggestion for C++20 ranges:

Ranges:

The ranges library provides a new way of working with ranges of elements. To use them,
you should include the header file. Let's look at ranges with an example. A
range is a sequence of elements having a beginning and an end. It provides a begin iterator
and an end sentinel. Consider the following vector of integers:

#include <vector>
int main()
{
    std::vector<int> elements{0, 1, 2, 3, 4, 5, 6};
}

Ranges accompanied by range adapters (the | operator) provide powerful functionality to
deal with a range of elements. For example, examine the following code:

#include <vector>
#include <ranges> //works in GCC 10.1(current)
int main()
{
    std::vector<int> elements{0, 1, 2, 3, 4, 5, 6};
    for (int current : elements | std::ranges::view::filter([](int e) { return
         e % 2 == 0; }))
  {
      std::cout << current << " ";
  }
}

[changkun]

@kassane Thank you so much!

[Peilix]

@changkun 方便发布一版中文的EPUB吗 dalao

[changkun]

@Peilix https://changkun.de/modern-cpp/epub/modern-cpp-tutorial-zh-cn.epub

[Sunbreak]

Any progress?

[changkun]

PR welcome