Enforce displaying Amount with trailing zeros

It is common to display bitcoins using trailing zeros upto 8 decimals.
This commit enforces:
- Displaying Amount in BTC with trailing zeroes by eight decimal places
  if a precision on the Amount is not specified.
- Displaying Amount in BTC upto the precision specified truncating the insignificant zeros.
- Displaying amount in BTC without any decimals if the remainder of the amount
  divided by the satoshis in 1 BTC is equal to zero using formula `satoshis.rem_euclid(Amount::ONE_BTC.to_sat()) != 0`

These are not breaking changes and all previous tests pass.

A testcase is added to for changes introduced.

Resolves: #2136

units/src/amount.rs

@@ -1064,8 +1064,15 @@ impl fmt::Debug for Amount {
 // Just using Bitcoin denominated string.
 impl fmt::Display for Amount {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        self.fmt_value_in(f, Denomination::Bitcoin)?;
-        write!(f, " {}", Denomination::Bitcoin)
+        let satoshis = self.to_sat();
+        let denomination = Denomination::Bitcoin;
+        let mut format_options = FormatOptions::from_formatter(f);
+
+        if f.precision().is_none() && satoshis.rem_euclid(Amount::ONE_BTC.to_sat()) != 0 {
+            format_options.precision = Some(8);
+        }
+
+        fmt_satoshi_in(satoshis, false, f, denomination, true, format_options)
     }
 }
 
@@ -2907,4 +2914,35 @@ mod tests {
             }
         }
     }
+
+    #[test]
+    #[cfg(feature = "alloc")]
+    fn trailing_zeros_for_amount() {
+        assert_eq!(format!("{}", Amount::ONE_SAT), "0.00000001 BTC");
+        assert_eq!(format!("{}", Amount::ONE_BTC), "1 BTC");
+        assert_eq!(format!("{}", Amount::from_sat(1)), "0.00000001 BTC");
+        assert_eq!(format!("{}", Amount::from_sat(10)), "0.00000010 BTC");
+        assert_eq!(format!("{:.2}", Amount::from_sat(10)), "0.0000001 BTC");
+        assert_eq!(format!("{:.2}", Amount::from_sat(100)), "0.000001 BTC");
+        assert_eq!(format!("{:.2}", Amount::from_sat(1000)), "0.00001 BTC");
+        assert_eq!(format!("{:.2}", Amount::from_sat(10_000)), "0.0001 BTC");
+        assert_eq!(format!("{:.2}", Amount::from_sat(100_000)), "0.001 BTC");
+        assert_eq!(format!("{:.2}", Amount::from_sat(1_000_000)), "0.01 BTC");
+        assert_eq!(format!("{:.2}", Amount::from_sat(10_000_000)), "0.10 BTC");
+        assert_eq!(format!("{:.2}", Amount::from_sat(100_000_000)), "1.00 BTC");
+        assert_eq!(format!("{}", Amount::from_sat(100_000_000)), "1 BTC");
+        assert_eq!(format!("{}", Amount::from_sat(40_000_000_000)), "400 BTC");
+        assert_eq!(
+            format!("{:.10}", Amount::from_sat(100_000_000)),
+            "1.0000000000 BTC"
+        );
+        assert_eq!(
+            format!("{}", Amount::from_sat(400_000_000_000_010)),
+            "4000000.00000010 BTC"
+        );
+        assert_eq!(
+            format!("{}", Amount::from_sat(400_000_000_000_000)),
+            "4000000 BTC"
+        );
+    }
 }