Multiple memory safety issues in insert_row

When inserting rows from an iterator at a particular index, toodee would shift
items over, duplicating their ownership. The space reserved for the new elements
was based on the len() returned by the ExactSizeIterator.

This could result in elements in the array being freed twice if the iterator
panics. Uninitialized or previously freed elements could also be exposed if the
len() didn’t match the number of elements.

These issues were fixed in commit ced70c17 by temporarily setting the length
of the array smaller while processing it and adding assertions on the number
of elements returned by the iterator.