Revise FromIterator for Decimal128Array to use Into instead of Borrow

[viirya]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Currently the element type of FromIterator for Decimal128Array is Borrow<Option<i128>. But Borrow<Option<i128> is hard to extend to other types e.g. Decimal128. Because the rust compiler doesn't allow us to implement Borrow<Option<i128> for Option<Decimal128> as Option is not defined in this crate.

Using Into trait seems more suitable and extensible.

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