Dynamic API almost done. Reading is complete.

Still need to flesh out the writing part of the API. init functions
are making it very messy at the moment.

fixMaybe.h

@@ -0,0 +1,11 @@
+#include "kj/common.h"
+#include <stdexcept>
+
+template<typename T>
+T fixMaybe(::kj::Maybe<T> val) {
+  KJ_IF_MAYBE(new_val, val) {
+    return *new_val;
+  } else {
+    throw std::invalid_argument("member was null");
+  }
+}