Refactor to use one module per concept

This makes code navigation easier.

Signed-off-by: Jacob Kiers <code@kiers.eu>

examples/list.rs

@@ -5,8 +5,10 @@ fn main() -> Result<()> {
     let cfg = BunqConfig::load()?;
 
     let cfg = cfg.install()?;
+
     let accs = cfg.monetary_accounts()?;
     println!("{:#?}", accs);
+
     let acc = &accs[0];
     let ps = cfg.payments(acc)?;
     println!("{:#?}", ps);