Remove Default implementation of MintMeltLimits

The default would have been min=0, max=0. This made any mint initialized with MintMeltLimits::default() to fail every mint and melt operation, because the amount was out of bounds.

crates/cdk-integration-tests/src/init_pure_tests.rs

@@ -167,7 +167,7 @@ pub async fn create_and_start_test_mint() -> anyhow::Result<Arc<Mint>> {
     mint_builder = mint_builder.add_ln_backend(
         CurrencyUnit::Sat,
         PaymentMethod::Bolt11,
-        MintMeltLimits::default(),
+        MintMeltLimits::new(1, 1_000),
         ln_fake_backend,
     );
 

crates/cdk/src/mint/builder.rs

@@ -242,8 +242,8 @@ impl MintBuilder {
     }
 }
 
-/// Mint Melt Limits
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default)]
+/// Mint and Melt Limits
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct MintMeltLimits {
     /// Min mint amount
     pub mint_min: Amount,
@@ -256,7 +256,7 @@ pub struct MintMeltLimits {
 }
 
 impl MintMeltLimits {
-    /// Create new [`MintMeltLimits`]
+    /// Create new [`MintMeltLimits`]. The `min` and `max` limits apply to both minting and melting.
     pub fn new(min: u64, max: u64) -> Self {
         Self {
             mint_min: min.into(),