Utility functions

[steinerkelvin]

We should be able to provide functions like benda.util.to_tree(xs: Iterable[T]) -> Tree[T] native-optimized, though I'm not sure on the interface format.

We should also be able to iterate over Tree and List easily and
native-optimizedly with functions like benda.util.from_tree(tree: Tree[T]) -> Iterator[T]:

tree = book.adts.Tree.Node(…, …, …)
for x in benda.util.iter_tree(tree):
   print(x)

I'm inclined to think we should not provide this __iter__ behavior directly on
the types of Tree.Node etc because this behavior is not that obvious and is
implemented as separate functions on Bend's side.

---

• benda.util.to_nat(int) -> Nat throws ValueError("Negative integer")
• benda.util.from_nat(Nat) -> int
• benda.util.to_string(str) -> String
• benda.util.from_string(String) -> str
• benda.util.to_list(Iterable[T]) -> List[T]
• benda.util.from_list(List[T]) -> list[T]
• benda.util.iter_list(List[T]) -> Iterator[T]
• benda.util.iter_tree(Tree[T]) -> Iterator[T]

Note: str conversion from and to String should probably also automatic, but we can't
do that for int ↔ Nat because they don't map directly.