Interest check: implement __bool__ for Result and Maybe

该 issue 没有指定具体的文件或测试。首先定位 Result 和 Maybe 的实现及其现有的类型收窄测试，然后检查如何区分 Success 和 Failure。Done 应包括已约定的两种类型的 bool 行为、对已揭示类型的覆盖，以及考虑与现有 truthiness 假设的兼容性。

Long before I found Returns, I implemented my own Result monad from scratch. It was a lot of fun, and right now it coexists with Returns as the library it's a part of is being incrementally deprecated. At some point, I decided it would be nice if I could do things like this:

result = some_function()  # Result[S, F]
if result:
    reveal_type(result)  # Success[S]
else:
    reveal_type(result)  # Failure[F]

It's worked great. I have my own analogue to is_successful, but with __bool__ I don't even have to import it. Also, it actually works better than my functions: my analogue is a pair of TypeGuard functions (TypeIs didn't exist yet), so even after narrowing the container type in the first if, the else would have no type information about the container. Cf., __bool__(self) -> Literal[False] and __bool__(self) -> Literal[True] make type narrowing work perfectly, and it feels very pythonic, imo.

As I update more and more code to use Returns, instead, I find this to be the one thing about my library that I actually miss. There are a lot of cases where I could reach for match for something as ergonomic, but most of the time I'm only interested in Success vs. Failure; match is overkill if I don't need to peak at the contained value.

The one drawback I anticipate to adding this feature would be the possibility of breaking any code that's relying on the default object.__bool__ behavior to differentiate a Failure from eg. a None... but hopefully, anybody using this library would have their Optionals wrapped in a Maybe already?