`NewResult` makes it easy to wrap a normal function call in a result
value.
`Err` can be used to check the error case of the result without
unpacking the entire thing.
`AndThen2` allows a caller to compose a function on two results values,
with the closure only executing if both values are non-error.
In this commit, we add a new result type, which is just Either[A, error]. This is useful as we can always pass around/accept a single value. We add an Unpack() method which is the most idiomatic way of handling errors in Go, as we revert back to (A, error) when necessary.
