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ARROW-12398: [Rust] remove redundant bound check in iterators

This PR removes the bound checks as discussed in #9994.

Furthermore I added `unsafe` versions of the `value` method to `PrimitiveArray` and `BooleanArray`. The `safe` marked methods are actually `unsafe`. This way we can slowly transition to explicitly using the `unsafe` variant and later make the "safe" one truly safe.

For the time being I also added a `debug_assert` bounds check in those "safe" methods that are `unsafe`. That way we at least get a panic in debug mode instead of UB in safe code.

Closes #10046 from ritchie46/iterator_bounds

Authored-by: Ritchie Vink <ritchie46@gmail.com>
Signed-off-by: Andrew Lamb <andrew@nerdnetworks.org>