Add solution for day 15 part 1

src/Day15.kt

@@ -0,0 +1,26 @@
+fun main() {
+    fun part1(input: List<String>): Int {
+        return input.first()
+            .split(',')
+            .fold(0) { acc, s ->
+                acc + s.hash()
+            }
+    }
+
+    fun part2(input: List<String>): Int {
+        return input.size  
+    }
+
+    // test if implementation meets criteria from the description, like:
+    val testInput = readInput("Day15_test")
+    check(part1(testInput) == 1320)
+
+    val input = readInput("Day15")
+    part1(input).println()
+    check(part2(testInput) == 0)
+    part2(input).println()
+}
+
+fun String.hash(): Int = toCharArray().fold(0) { sum, char ->
+    ((sum + char.code) * 17) % 256
+}