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ARROW-10449 [Rust] Make Dictionary::keys be an array

This PR:

1. Makes `DictionaryArray::keys` be an `PrimitiveArray`.
2. Removes `NullIter` and many of the `unsafe` code that it contained
3. Simplifies the parquet writer implementation around dictionaries
4. Indirectly removes a bug on NullIter's `DoubleEndedIterator` on which the implementation was not following the spec with respect to `end == current` (compare with implementation of `DoubleEndedIterator` for std's `Vec`'s `IntoIter`).
5. Fixes error in computing the size of a dictionary, which was double-counting `data` and `values` (`values` is a reference to `data`)
6. Adds check that the dictionary's ArrayData's datatype matches `T::DATA_TYPE`.

Since `NullIter` was not being directly tested, no tests were removed.

This is backward incompatible and the migration requires replacing `dict.keys()`  by `dict.keys().into_iter()` whenever the user's intention is to use `keys()` as an iterator.

Closes #8561 from jorgecarleitao/dictionary_clean

Authored-by: Jorge C. Leitao <jorgecarleitao@gmail.com>
Signed-off-by: Neville Dipale <nevilledips@gmail.com>