A minimal std::expected<T, E>

[deanberris]

This change includes a minimal implementation of the std::expected<T, E>
proposal in P0323R3 (http://wg21.link/P0323R3) which is hosted in the
cppnetlib namespace.

In the process we also document some of our guidelines on the structure
of the repository, on naming files, directory structure, and dependency
requirements.

Still TODO:

• Update documentation on using vcpkg on dependencies.
• Work our way up to a minimal interface for a connecton type, using our
  std::expected utility type.

This is related to the work in #5 for implementing a minimal
connection type suitable as a building block for higher level protocol
implementations.

[deanberris]

Hi @glynos -- this is now ready for a look. Some thoughts:

• Is there a way to make the CI learn to use vcpkg?
• Aside from the inline documentation (using Doxygen-style comments), I'd appreciate a look through at the spec and the implementation.
• I can start making progress on the connection type on top of this, and incrementally start adding functions when it turns out we need some of them. I skilled the emplace and in-place construction support, but those are trivial to add.

[AngleNet]

Suggests to separate test code into another directory lead to more clean layout.