C++11/14/17 std::expected with functional-style extensions

 Single header implementation of std::expected with functional-style extensions.
 .
 std::expected is proposed as the preferred way to represent object which will
 either have an expected value, or an unexpected value giving information about
 why something failed. Unfortunately, chaining together many computations which
 may fail can be verbose, as error-checking code will be mixed in with the
 actual programming logic. This implementation provides a number of utilities to
 make coding with expected cleaner.