feat: add pararell iterators poc

rusty_iterators/iterators/pararell/RATIONALE.md

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+# The general idea behind pararell iteration
+
+## Sources
+
+- https://smallcultfollowing.com/babysteps/blog/2016/02/19/parallel-iterators-part-1-foundations/
+- https://smallcultfollowing.com/babysteps/blog/2016/02/25/parallel-iterators-part-2-producers/
+- https://smallcultfollowing.com/babysteps/blog/2016/11/14/parallel-iterators-part-3-consumers/
+- https://geo-ant.github.io/blog/2022/implementing-parallel-iterators-rayon/

rusty_iterators/iterators/pararell/_consumers.py

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+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from typing import TYPE_CHECKING, Self
+
+if TYPE_CHECKING:
+    from ._folders import FolderInterface
+    from ._reducers import ReducerInterface
+
+
+class ConsumerInterface[T, R](ABC):
+    __slots__ = ()
+
+    @abstractmethod
+    def as_folder(self) -> FolderInterface[T, R]:
+        raise NotImplementedError
+
+    @abstractmethod
+    def split(self, n: int) -> tuple[Self, Self, ReducerInterface[R]]:
+        raise NotImplementedError

rusty_iterators/iterators/pararell/_folders.py

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+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from typing import Self
+
+
+class FolderInterface[T, R](ABC):
+    __slots__ = ()
+
+    @abstractmethod
+    def consume(self, item: T) -> Self:
+        raise NotImplementedError
+
+    @abstractmethod
+    def complete(self) -> R:
+        raise NotImplementedError

rusty_iterators/iterators/pararell/_iter.py

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+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+
+
+class PararellIterInterface[T](ABC):
+    __slots__ = ()
+
+    @abstractmethod
+    def size_hint(self) -> int:
+        raise NotImplementedError

rusty_iterators/iterators/pararell/_producers.py

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+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from typing import TYPE_CHECKING, Self
+
+if TYPE_CHECKING:
+    from rusty_iterators.iterators._sync import IterInterface
+
+
+class ProducerInterface[T](ABC):
+    __slots__ = ()
+
+    @abstractmethod
+    def as_iter(self) -> IterInterface[T]:
+        raise NotImplementedError
+
+    @abstractmethod
+    def split(self, n: int) -> tuple[Self, Self]:
+        raise NotImplementedError

rusty_iterators/iterators/pararell/_reducers.py

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+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+
+
+class ReducerInterface[R](ABC):
+    __slots__ = ()
+
+    @abstractmethod
+    def reduce(self, left: R, right: R) -> R:
+        raise NotImplementedError