Implement Maybe monad and custom mypy plugin about returns HOT 8 CLOSED

Moreover it is possible to use Optional. And we do not rely on it internally.
And it can be replaced with Result.

from returns.

We have to wait until python/typing#614 is resolved.
And then re-add this monad to the project with correct type.

from returns.

Closed by accident.

from returns.

We can probably try to write custom mypy plugin to solve:

1. decorator @safe and @impure untyped parameters
2. Maybe.new() type issue

Let's evaluate how hard it is to do.

from returns.

Refs #88

from returns.

It is also became clear to me that we should always return Maybe[A] from Maybe.new. So, there's no issue in typing.

from returns.

We can start working on Maybe monad!

from returns.

Closed with cf36f26

from returns.