feat(corelib): Iterator::fold

[MagisterDallis]

Implements Iterator::fold.

Not using a traditional for-loop technique as it's still buggy, as seen in advance_by.

[reviewable-StarkWare]

This change is 

[orizi]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @MagisterDallis)

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corelib/src/iter/traits/iterator.cairo line 192 at r1 (raw file):

            },
            Option::None => { accum },
        }

Suggestion:

        match Self::next(ref self) {
            Option::None => init,
            Option::Some(x) => Self::fold(ref self, f(init, x), f),
        }

[MagisterDallis]

Reviewable status: 1 of 2 files reviewed, 1 unresolved discussion (waiting on @orizi)

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corelib/src/iter/traits/iterator.cairo line 192 at r1 (raw file):

            },
            Option::None => { accum },
        }

Done.

[orizi]

Note it does properly work now - but the recursive implementation is still likely to be a bit more efficient.

Reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @MagisterDallis)

[orizi]

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @MagisterDallis)

---

a discussion (no related file):
@gilbens-starkware for 2nd eye.

[gilbens-starkware]

Reviewed 1 of 2 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @MagisterDallis)

[orizi]

Reviewed 2 of 2 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @MagisterDallis)