implement basic Java 8 lambda support

The two big pieces here are basic invokedynamic support and a working
version of LambdaMetaFactory.metafactory.  The latter works by
dynamically building a synthetic class with three methods: a static
factory method, a constructor for the factory method to call, and a
method to satisfy the requested interface which defers to the
specified MethodHandle.

This work relies heavily on Avian's specific MethodType and
MethodHandle implementations, which provide extra, non-standard
features to make code generation easier.  That means we'll probably
need to use Avian's versions of java.lang.invoke.* even when building
with the OpenJDK or Android class libraries.

classpath/java/lang/invoke/MethodHandles.java

@@ -1,5 +1,18 @@
 package java.lang.invoke;
+
 public class MethodHandles {
   public static class Lookup {
+    final avian.VMClass class_;
+    private final int modes;
+
+    private Lookup(avian.VMClass class_, int modes) {
+      this.class_ = class_;
+      this.modes = modes;
+    }
+
+    public String toString() {
+      return "lookup[" + avian.SystemClassLoader.getClass(class_) + ", "
+        + modes + "]";
+    }
   }
 }