Result should allow errors of any type

[nozzlegear]

Not all Result use-cases need an actual Error object to represent the error. Sometimes you might just want to return a string. The Result monad should be refactored to accept two generic times: one for the result and one for the error. It should use an underlying union type to determine what is a result and what is an error.

// A function that returns a Result containing either the parsed number, or an error message.
function parseNumber(str: string): Result<number, string> {
    const parsed = parseInt(str):

    if (isNaN(parsed)) {
        return Result.ofError("Parsed string was not a number!");
    }

    return Result.ofValue(parsed);
}

[nozzlegear]

Any Result function that could conceivably throw an error (e.g. Result.ofFunction or Result.ofPromise) should require a callback function that would specifically map the error into something. We want to guarantee that the error value of a Result is what the compiler says it is.

[nozzlegear]

Result.mapError should also be added to map the error into something else.