Implement AsRef for ICS-20 Amount type (#685)

* make inner type public

* changelog

* change to AsRef instead of `pub` field

Author: plafer

.changelog/unreleased/improvements/684-fix-Amount.md

@@ -0,0 +1,2 @@
+- Token transfer: Make `Amount` type less restrictive
+  ([#684](https://github.com/cosmos/ibc-rs/issues/684))

crates/ibc/src/applications/transfer/amount.rs

@@ -21,6 +21,12 @@ impl Amount {
     }
 }
 
+impl AsRef<U256> for Amount {
+    fn as_ref(&self) -> &U256 {
+        &self.0
+    }
+}
+
 impl FromStr for Amount {
     type Err = TokenTransferError;