Orthogonalize re-indexing for FastSubArrays (#53369)
By separating out the re-indexing step for `FastSubArray`s and
specializing this for `FastContiguousSubArray`s, we don't need to define
specialized `getindex`, `setindex!` and `isassigned` for
`FastContiguousSubArray`s anymore. The fallback method for
`FastSubArray`s will correctly handle the special case.
