Resolves #3274

The implementation in vector_algorithm.cpp is similar to the forward find, except:

• Advancing the _Last to negative direction
  • For negative advance, _Rewind_bytes was introduced. This helps avoiding casting size_t to ptrdiff_t and running into potential UB due to overflow when the range is more than half addressable space
  • The existing _Advance_bytes was made template to fix pre-existing potentially large size_t to ptrdiff_t conversion
• Advance before the indirection, so that start with past-the-last pointer and stop on first
• Still returning last pointer on failure, save it for such case
• _BitScanForward -> _BitScanReverse. _lzcnt_u32 -> _tzcnt_u32
  • both _BitScanForward and_BitScanReverse index from the least significant bit, so no changes here
  • _tzcnt_u32 index from most significant bit, so reverse using 31 - x
  • AVX2 imply _tzcnt_u32 is present, same as _lzcnt_u32. We have precedent in <__msvc_bit_utils.hpp>

The integration in <algorithm> is similar to ranges::find, except that we don't support unsized ranges (in this regard, similar to ranges::count).

The test uses the same random data as for forward find. I made sure it covers all branches of the algorithm.