Add trivial fib benchmark.

doc/implicit fields.txt

@@ -0,0 +1,29 @@
+Q: Can fields be implicitly declared?
+
+The idea is that just using a name starting with "_" somewhere in a class
+automatically defines a field with that name. Implicit fields are particularly
+nice because it means they don't have to be defined all before methods. (Since
+we have a single-pass compiler, we would have to otherwise a method could
+only refer to previously defined fields.)
+
+One potential problem is with nested classes. This is more important if we
+consider a module effectively a class. Consider:
+
+  class Outer {
+    foo {
+      _blah = "value"
+    }
+
+    class Inner {
+      io.write(_blah) // Does this declare field in Inner, or access Outer?
+    }
+  }
+
+Looking at this, though, I think there's already a question how referring to an
+outer field would work. Having an instance of Inner doesn't imply you also have
+an instance of Outer. We definitely don't want to recapitulate inner classes
+in Java.
+
+Q: What about static fields?
+
+A: Different naming convention? __foo?