Implement Maybe monad and custom mypy plugin

[sobolevn]

Currently it is impossible to use this monad due to this bug in typing:

x: Optional[int]
Monad.new(x)  # ?

Type will be: Some[Union[int, None]]
, not Union[Some[int], Nothing]
Related:

returns/returns/maybe.pyi

Lines 10 to 24 in edb8ae2

	_ContainerType = TypeVar('_ContainerType', bound=Container)
	_ValueType = TypeVar('_ValueType')
	_NewValueType = TypeVar('_NewValueType')
	class Maybe(GenericContainerOneSlot[_ValueType], metaclass=ABCMeta):
	@overload
	@classmethod
	def new(cls, inner_value: Literal[None]) -> 'Nothing': # type: ignore
	...
	@overload # noqa: F811
	@classmethod
	def new(cls, inner_value: _ValueType) -> 'Some[_ValueType]':
	...

[sobolevn]

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

[sobolevn]

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

[sobolevn]

Closed by accident.

[sobolevn]

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.

[sobolevn]

Refs #88

[sobolevn]

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

[sobolevn]

We can start working on Maybe monad!

[sobolevn]

Closed with cf36f26