Refactor toRoman function

Repetition of the decreasing powers of ten suggested a refactor.
This called for an additional function to map a base unit (power of ten)
to its single-character Roman symbol.

src/main/kotlin/RomanNumerals.kt

@@ -1,11 +1,21 @@
 fun Int.toRoman(): String {
-    if (this >= 1000) {
-        return "M".repeat(this / 1000)
-    } else if (this >= 100) {
-        return "C".repeat(this / 100)
-    } else if (this >= 10) {
-        return "X".repeat(this / 10)
-    } else {
-        return "I".repeat(this)
+    val powersOfTen = listOf(1000, 100, 10, 1)
+    powersOfTen.forEach { unit ->
+        if (this >= unit) {
+            return symbolFor(unit).repeat(this / unit)
+        }
     }
-}
+    throw RomanNumeralException("Can't yet convert ${this} to Roman numerals")
+}
+
+fun symbolFor(unit: Int): String {
+    return when(unit) {
+        1000 -> "M"
+        100 -> "C"
+        10 -> "X"
+        1 -> "I"
+        else -> throw RomanNumeralException("No mapping to symbol for ${unit}")
+    }        
+}
+
+class RomanNumeralException(message: String) : RuntimeException(message)