kvstore: iter_mut and for tables

[nrc]

This PR replace the old for_each_mut API with mutable iterator API (slightly different for different kinds of tables/accesses). The main difficulty here (as well as some interesting lifetime/borrow checker issues) is that after mutation we have to rebuild indexes. That requires some tracking. I've opted to track individual keys since that will be more efficient if a small minority of values are mutated (which might be the case for indexes in particular). However, if the common case is that a whole table is modified then rebuilding the whole index might be more efficient. The other interesting thing is that for the raw API, we need to do the implicit commit somewhere. I explored having this in the dtor of an iterator, but this ended up needing some complex self-referential stuff, so there is a with_iter_mut function rather than iter_mut. If we really need the latter, we can probably do it, but it will require some unsafe code or pinning or something.

Based on #263
Closes #217

AI decl: used Claude to generate tests and for implementing mutation tracking in TableIteratorMut (following similar code for IndexedIteratorMut). Code has been reviewed and refactored.