Fill out the Result API

[danakj]

https://doc.rust-lang.org/stable/std/result/enum.Result.html

There is a TODO in the result.h header:

// TODO: Result<void, E>
// TODO: Result<T&, E&>
// TODO: and, and_then, as_mut, as_ref, cloned, copied, expect, expect_err,
// etc...
// https://doc.rust-lang.org/std/result/enum.Result.html

Like Option, we want to clone the full API of Result.

We also need a way to access (at least) the result without turning the Result into an rvalue, like operator* and operator-> on Option.