use a formatter to display Btc/MilliBtc amounts

Display a fixed number of digits after the comma, instead of the default which may even use the exponent display occasionnally.

Please double check the `MilliBtc` case as it's easy to get it wrong.

Author: pm47

src/main/scala/fr/acinq/bitcoin/scalacompat/BtcAmount.scala

@@ -52,7 +52,7 @@ case class MilliBtc(private val underlying: BigDecimal) extends BtcAmount with O
   def toBigDecimal: BigDecimal = underlying
   def toDouble: Double = underlying.toDouble
   def toLong: Long = underlying.toLong
-  override def toString = s"$underlying mBTC"
+  override def toString = f"$underlying%1.5f mBTC"
   // @formatter:on
 }
 
@@ -82,12 +82,12 @@ case class Btc(private val underlying: BigDecimal) extends BtcAmount with Ordere
   def toBigDecimal: BigDecimal = underlying
   def toDouble: Double = underlying.toDouble
   def toLong: Long = underlying.toLong
-  override def toString = s"$underlying BTC"
+  override def toString = f"$underlying%1.8f BTC"
   // @formatter:on
 }
 
 object BtcAmount {
   val Coin = 100000000L
   val Cent = 1000000L
   val MaxMoney: Double = 21e6 * Coin
-}
+}