fix: add intrinsics to header for clang-msvc compilation

[IlyaGredasov]

Such code can be compiled in clang wsl:

[/mnt/d/Programming/C++]$ cat test.cpp
#include <slimcpplib/long_int.h>
#include <slimcpplib/long_io.h>

using namespace slim::literals;

int main() {
        const slim::int256_t u = -10000_si256 << 100;
        std::cout << u << std::endl;
}
[/mnt/d/Programming/C++]$ clang++ test.cpp -std=c++20 -I slimcpplib/include
[/mnt/d/Programming/C++]$ ./a.out
-12676506002282294014967032053760000
[/mnt/d/Programming/C++]$

But msvc clang doesn't find intrinsics for compilation:

PS D:\Programming\C++> cat .\test.cpp
#include <slimcpplib/long_int.h>
#include <slimcpplib/long_io.h>

using namespace slim::literals;

int main() {
        const slim::int256_t u = -10000_si256 << 100;
        std::cout << u << std::endl;
}
PS D:\Programming\C++> clang++ .\test.cpp -std=c++20 -I .\slimcpplib\include
In file included from .\test.cpp:1:
In file included from .\slimcpplib\include\slimcpplib/long_int.h:39:
In file included from .\slimcpplib\include\slimcpplib\long_uint.h:35:
In file included from .\slimcpplib\include\slimcpplib\long_math_long.h:42:
.\slimcpplib\include\slimcpplib\long_math_msvc.h:231:12: error: use of undeclared identifier '_addcarry_u8'
  231 |     return _addcarry_u8(0, value1, value2, &value1);
      |            ^~~~~~~~~~~~
.\slimcpplib\include\slimcpplib\long_math_msvc.h:239:12: error: use of undeclared identifier '_addcarry_u16'
  239 |     return _addcarry_u16(0, value1, value2, &value1);
      |            ^~~~~~~~~~~~~
.\slimcpplib\include\slimcpplib\long_math_msvc.h:267:12: error: use of undeclared identifier '_addcarry_u8'
  267 |     return _addcarry_u8(carry, value1, value2, &value1);
      |            ^~~~~~~~~~~~
.\slimcpplib\include\slimcpplib\long_math_msvc.h:274:12: error: use of undeclared identifier '_addcarry_u16'
  274 |     return _addcarry_u16(carry, value1, value2, &value1);
      |            ^~~~~~~~~~~~~
.\slimcpplib\include\slimcpplib\long_math_msvc.h:322:12: error: use of undeclared identifier '_subborrow_u8'
  322 |     return _subborrow_u8(0, value1, value2, &value1);
      |            ^~~~~~~~~~~~~
.\slimcpplib\include\slimcpplib\long_math_msvc.h:330:12: error: use of undeclared identifier '_subborrow_u16'
  330 |     return _subborrow_u16(0, value1, value2, &value1);
      |            ^~~~~~~~~~~~~~
.\slimcpplib\include\slimcpplib\long_math_msvc.h:358:12: error: use of undeclared identifier '_subborrow_u8'
  358 |     return _subborrow_u8(borrow, value1, value2, &value1);
      |            ^~~~~~~~~~~~~
.\slimcpplib\include\slimcpplib\long_math_msvc.h:366:12: error: use of undeclared identifier '_subborrow_u16'
  366 |     return _subborrow_u16(static_cast<uint8_t>(borrow), value1, value2, &value1);
      |            ^~~~~~~~~~~~~~
In file included from .\test.cpp:1:
In file included from .\slimcpplib\include\slimcpplib/long_int.h:39:
.\slimcpplib\include\slimcpplib\long_uint.h:1032:32: warning: identifier '_ui128' preceded by whitespace in a literal
      operator declaration is deprecated [-Wdeprecated-literal-operator]
 1032 | constexpr uint128_t operator"" _ui128() noexcept
      |                     ~~~~~~~~~~~^~~~~~
      |                     operator""_ui128
.\slimcpplib\include\slimcpplib\long_uint.h:1038:32: warning: identifier '_ui256' preceded by whitespace in a literal
      operator declaration is deprecated [-Wdeprecated-literal-operator]
 1038 | constexpr uint256_t operator"" _ui256() noexcept
      |                     ~~~~~~~~~~~^~~~~~
      |                     operator""_ui256
In file included from .\test.cpp:1:
.\slimcpplib\include\slimcpplib/long_int.h:590:31: warning: identifier '_si128' preceded by whitespace in a literal
      operator declaration is deprecated [-Wdeprecated-literal-operator]
  590 | constexpr int128_t operator"" _si128() noexcept
      |                    ~~~~~~~~~~~^~~~~~
      |                    operator""_si128
.\slimcpplib\include\slimcpplib/long_int.h:596:31: warning: identifier '_si256' preceded by whitespace in a literal
      operator declaration is deprecated [-Wdeprecated-literal-operator]
  596 | constexpr int256_t operator"" _si256() noexcept
      |                    ~~~~~~~~~~~^~~~~~
      |                    operator""_si256
4 warnings and 8 errors generated.
PS D:\Programming\C++>

[devatrun]

It has now been verified that the code is successfully compiled for clang 11 (and higher), gcc 11 (and higher) and msvc 14 (and higher). If you need to compile using clang in msvc mode, then you must first set the desired environment and environment variables, for example using vcvarsall.bat. This is necessary so that the correspondent headers can be found during compilation.
Unfortunately, your proposal will lead to fragility and heavy reliance on Microsoft's implementation.