fix exact-size iterator contracts for rows and columns

[ginavalent]

Fix ExactSizeIterator implementations for:

• RowIterator
• ColumnIter
• ColumnIterMut

These iterators previously returned the total number of rows or columns from len(), even after elements had been consumed. They also inherited the default non-exact Iterator::size_hint() implementation.

Each iterator now reports the number of remaining elements through an exact size_hint():

(remaining, Some(remaining))

The ExactSizeIterator implementations now use the default len() implementation derived from size_hint().

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[ginavalent]

signed.

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