add a bunch of classes to classpath and flesh out a few existing ones

classpath/java/lang/Runtime.java

@@ -0,0 +1,17 @@
+package java.lang;
+
+public class Runtime {
+  private static final Runtime instance = new Runtime();
+
+  private Runtime() { }
+
+  public static Runtime getRuntime() {
+    return instance;
+  }
+
+  public native void loadLibrary(String name);
+
+  public native void gc();
+
+  public native void exit(int code);
+}