Constexpr scope guards

Constexpr-ized version of scope guards in LFTS v3. Single-header implementation.

Currently referenced version: N4948

Other documentation(s):

• cppreference

Requires:

• C++20 constraints (requires-clause) (reference)
• C++20 std::is_constant_evaluated (reference)
• C++20 constexpr destructors (reference)
• C++20 [[no_unique_address]] (reference)
• Numerous C++17/14/11 features

Modifications from the LFTS v3 version:

• utilities are in namespace ::fvs instead of ::std::experimental
• all member and non-member functions are constexpr-ized

Test cases are still missing for now. (2022-09-05)