`contiguous_iter` should be a compile error if not using table storage strategy

## What problem does this solve or what need does it fill?

Using the wrong storage and attempting to use these methods results in a runtime error, which is suboptimal.

## What solution would you like?

We can improve the API by making these iteration methods not return a result and instead gate the trait implementation on the GAT.

## What alternative(s) have you considered?

N/A

## Additional context

This is a new feature in 0.19, so if this is fixed in the RC, that would be cool.

Start by locating the contiguous_iter methods and the trait implementation gated by the storage-strategy GAT. Compare the table and non-table storage paths, then verify that invalid uses fail at compile time while valid table iteration remains available.

Written by the indexing model from the issue text.