cxxomfort: C++20 Backports

cxxomfort rel.20210622

Simple backports for C++ - http://ryan.gulix.cl/fossil.cgi/cxxomfort/

Backports from C++20.

A listing of some of the features backported:

From core: Attributes likely and unlikely .
From core: Attribute nodiscard with message.
From <algorithm>: shift_left() , shift_right() .
From <algorithm>: Consistent container erasure.
From <algorithm>: constexpr for more functions.
From <bit>: New header.
From <bit>: std::endian.
From <functional>: bind_front() , unwrap_reference .
From <iterator>: ssize() .
From <numeric>: midpoint() .
From <string> , string_view: starts_with() , ends_with() .
From <type_traits>: type_identity , remove_cvref .
From <type_traits>: is_bounded_array , is_unbounded_array .
From <utility>: Safe integral comparisons.

See also: C++17 Backports C++23 Backports .