Move stdlib to top-level lazy property

When using the REPL it would otherwise be reloaded every line.

src/main/kotlin/eu/jameshamilton/klox/Main.kt

@@ -92,11 +92,8 @@ fun runPrompt() {
     }
 }
 
-fun parse(code: String): Program? {
-    hadError = false
-    hadRuntimeError = false
-
-    val stdlib = readFiles("/klox/")
+private val stdlib by lazy {
+    readFiles("/klox/")
         .filter { it.fileName.name.endsWith(".lox") }
         // ensure that Object.lox is loaded first since it defines the base class for all other classes
         .sortedBy { it.fileName.name != "Object.lox" }
@@ -110,6 +107,11 @@ fun parse(code: String): Program? {
         .map { Parser(it).parse() }
         .toList()
         .reduce { stdlib, lib -> stdlib + lib }
+}
+
+fun parse(code: String): Program? {
+    hadError = false
+    hadRuntimeError = false
 
     val scanner = Scanner(code)
     val tokens = scanner.scanTokens()