Consistent Integer Types

example of using, to avoid mixed usage of std::uint/std::int and uint/int...
```cpp
using u64 = std::uint64_t;
using u32 = std::uint32_t;
using u16 = std::uint16_t;
using u8  = std::uint8_t;

using i64 = std::int64_t;
using i32 = std::int32_t;
using i16 = std::int16_t;
using i8  = std::int8_t;

using usize = std::size_t;
using isize = std::ptrdiff_t;

#if defined(__GNUC__) && defined(IS_64BIT)
__extension__ using u128 = unsigned __int128;
__extension__ using i128 = signed __int128;
#endif
```

closes https://github.com/official-stockfish/Stockfish/pull/6874

No functional change

src/position.cpp

@@ -252,7 +252,7 @@ Position::set(const string& fenStr, bool isChess960, StateInfo* si) {
             if (file >= FILE_NB)
                 return PositionSetError("Invalid FEN. Invalid file reached.");
 
-            const size_t idx = PieceToChar.find(token);
+            const usize idx = PieceToChar.find(token);
             if (idx == string::npos)
                 return PositionSetError(std::string("Invalid FEN. Invalid piece: ")
                                         + std::string(1, token));