<algorithm>: algorithms should use TMP to provide optimized implementation for vector<bool> iterators

MS's STL already uses metaprogramming to help the optimizer produce
optimized code. As an example, std::fill delegates to std::memset
if safe, which has the advantage of guaranteed good performance in
Debug mode, as well as not having to rely on the optimizer to salvage
reasonable ASM out of the actual code.

I would like optimizations like this to also happen when calling various
algorithms on std::vector<bool>, which currently does not happen.

As an example, this code

void clean_out(std::vector<bool>& vec) {
    std::fill(vec.begin(), vec.end(), false);
}

should end up calling memset, rather than doing whatever this is.

Other algorithms that could be optimized:

• std::copy
• std::count
• std::find
• std::equal
• possibly others?

This would also improve the performance of std::vector<bool>'s member functions, as it internally uses std algorithms.

Start with the std::fill example for std::vector iterators and compare its generated code with the linked Godbolt example. Review the proposed std::copy, std::count, std::find, and std::equal cases; done means determining and implementing suitable optimized paths, including the affected vector member functions.