* We now separate the stored and exported multi index type
  `MultiIndexStorage` and `MultiIndex` in the `LocalView`.
* `MultiIndexStorage` will always have dynamic size and may need
  a larger capacity compared to `MultiIndex`s size.
* If it is clear that the multi indices generated by the `PreBasis`
  have a static length, the`MultiIndex` has static size, while
  the `MultiIndexStorage` is an `OverflowArray<MultiIndex, ...>`.
  Otherwise the `MultiIndex` is a `ReservedVector`.
* If the size of multi indices is guaranteed to be one, then the
  `MultiIndex` can be cast to the plain index to avoid using `[0]`
  in simple cases.