StorageVec Iterator

[davebryson]

Description

Added support for Iterator/IntoIterator to StorageVec

Checklist

• I have documented these changes where necessary.
• I have read the DCO and ensured that these changes comply.
• I assign this work under its open source licensing.

[rory-ocl]

Took a look at this. The approach is reasonable and works correctly for common value types, but there are a few things that would need addressing before merge:

Must fix:

• The slot/offset calculation in Iter::next() duplicates the logic from StorageVec::index_slot(). This should reuse the existing method or extract a shared helper — if the packing logic is ever updated in one place, the other will silently diverge.

Should fix:

• Missing size_hint() / ExactSizeIterator implementation. The iterator knows its exact length upfront (self.len - self.idx), so .collect::<Vec<_>>() can't pre-allocate without this.
• The Iter struct should be re-exported from the storage module for discoverability.

Minor:

• The into_iter test name is slightly misleading — it tests &StorageVec iteration via auto-ref, not a consuming iterator.

Not including this in the upcoming 0.10.3 release, but happy to revisit once these are addressed.