Replace Result with std::expected

[danstiner]

The Result type has served us well but as Hayden pointed out it would be nice to have an error code instead of just a message string.

Rather than extend Result I'd suggest moving to std::expected. It didn't quite make the C++20 standard but the feature is basically ready and there is a nice and popular compat library we can use until the next standard comes out: https://github.com/TartanLlama/expected