feat(corelib): Iterator::Sum

[MagisterDaIlis]

Sums an iterator

[reviewable-StarkWare]

This change is 

[orizi]

Reviewed 1 of 5 files at r1, all commit messages.
Reviewable status: 1 of 5 files reviewed, 1 unresolved discussion (waiting on @MagisterDaIlis)

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

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corelib/src/test/iter_test.cairo line 83 at r1 (raw file):

    let mut iter = array![].into_iter();
    let sum: usize = iter.sum();
    assert_eq!(sum, 0);

Suggestion:

    assert_eq!(array![1, 2, 3].into_iter().sum(), 6);
    assert_eq!(array![].into_iter().sum(), 0);

[orizi]

Reviewed 3 of 5 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @MagisterDaIlis)