Add spent trait to improve sent_and_received()

Describe the enhancement Add a TxAmountSpent trait to improve how transaction spending amounts are calculated. The trait will add a .spent() method to tuples returned by sent_and_received(), calculating total amount spent including fees.

Use case

// Current: Manual calculation needed
let (sent, received) = wallet.sent_and_received(&tx);
let fee = wallet.calculate_fee(&tx)?;
let total = sent - received + fee; // Error-prone 

// Proposed: Clean trait implementation
let (sent, received) = wallet.sent_and_received(&tx);
let total = (sent, received, fee).spent(); // Clear and consistent

Additional context

• Non-breaking change that extends existing functionality
• Makes transaction cost calculations more intuitive
• Helps prevent calculation errors
• Small, focused improvement suitable for a first-time contributor