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/MethodHandle.java

@@ -1,3 +1,23 @@
 package java.lang.invoke;
-public abstract class MethodHandle {
+
+public class MethodHandle {
+  private final ClassLoader loader;
+  final avian.VMMethod method;
+  private volatile MethodType type;
+  
+  MethodHandle(ClassLoader loader, avian.VMMethod method) {
+    this.loader = loader;
+    this.method = method;
+  }
+  
+  public String toString() {
+    return new java.lang.reflect.Method(method).toString();
+  }
+
+  public MethodType type() {
+    if (type == null) {
+      type = new MethodType(loader, method.spec);
+    }
+    return type;
+  }
 }