Add Go 1.23 iterators to collection types

[stringintech]

This PR adds idiomatic Go 1.23 iterators (iter.Seq) to collection types throughout the library. Each type with indexed access now provides three iterator methods following a consistent pattern:

• Items() - iterate over all items
• ItemsRange(from, to) - iterate over a specific range [from, to)
• ItemsFrom(from) - iterate from a starting index to the end

The iterators are added to: Block (transactions), Chain (block tree entries), Transaction (inputs and outputs), BlockSpentOutputs (transaction spent outputs), and TransactionSpentOutputs (coins).

Example: Iterating over block transactions

Previous approach:

for i := uint64(0); i < block.CountTransactions(); i++ {
    tx, err := block.GetTransactionAt(i)
    if err != nil {
        return err
    }
    // process tx
}

New approach:

for tx := range block.Transactions() {
    // process tx
}

Example: Iterating over chain entries from a specific height

Previous approach:

for h := startHeight; h <= chain.GetHeight(); h++ {
    entry := chain.GetByHeight(h)
    if entry == nil {
        break
    }
    // process entry
}

New approach:

for entry := range chain.EntriesFrom(startHeight) {
    // process entry
}