The iterator is implemented using const generics. It implements the
traits `Iterator`, `DoubleEndedIterator`, `ExactSizeIterator`,
`FusedIterator` and `TrustedLen`. It also contains a public method
`new` to create it from an array.
`IntoIterator` was not implemented for arrays yet, as there are still
some open questions regarding backwards compatibility. This commit
only adds the iterator impl and does not yet offer a convenient way
to obtain that iterator.
