Log Message:
devel: Add tl-expected

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.