Better SIFT in the main repository

The patent on one of the most popular feature detection algorithm, SIFT, expired on 2020/03/06. So, we can move the implementation from the repository for non-free modules (opencv_contrib/xfeatures2d) to the main OpenCV repository (opencv/features2d). Besides, there are platform-specific SIMD intrinsics in the implementation and currently the calculation is not deterministic due to floating-point operations, so I will make those codes more general and stable. An extension of the SIFT algorithm, A-SIFT, will also be implemented in C++ on the basis of its Python implementation in the Python modules.