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GH-34893: [C++] Fix run-end encoded array iterator issues that manifest on backwards iteration (#34896)

### Rationale for this change

The value of `RunEndEncodedArraySpan::Iterator::physical_pos_` from the instance returned by `RunEndEncodedArraySpan::end()` is smaller (by 1) than the results of successive applications of `operator++()` until `logical_pos_` reaches the logical length of the run-end encoded array.

To support backwards iteration, the `Iterator` returned by `RunEndEncodedArraySpan::end()` should be well-formed.

There was no `operator--()` in `RunEndEncodedArraySpan::Iterator` before this PR, so one can't go backwards from `end()` and this latent bug doesn't manifest. 

### What changes are included in this PR?

 - Addition of `RunEndEncodedArraySpan::Iterator::operator--()`
 - Addition of `MergedRunsIterator::operator--()`
 - Rename `isEnd()` to `is_end()` (it was a code-style violation)
 - More test cases

### Are these changes tested?

By existing and new unit tests.

### Are there any user-facing changes?

A public function -- `isEnd` -- was renamed, but this code hasn't been released yet, so it's OK.
* Closes: #34893

Authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>