Don't delete default dynamic builder constructors, because this makes things…

Don't delete default dynamic builder constructors, because this makes things difficult for the Python wrappers.

c++/src/capnp/dynamic-test.c++

@@ -444,9 +444,10 @@ TEST(DynamicApi, BuilderAssign) {
   auto root = builder.initRoot<DynamicStruct>(Schema::from<TestAllTypes>());
 
   // Declare upfront, assign later.
+  // Note that the Python implementation requires defaulted constructors.  Do not delete them!
   DynamicValue::Builder value;
-  DynamicStruct::Builder structValue = nullptr;
-  DynamicList::Builder listValue = nullptr;
+  DynamicStruct::Builder structValue;
+  DynamicList::Builder listValue;
 
   value = root.get("structField");
   structValue = value.as<DynamicStruct>();

c++/src/capnp/dynamic.h

@@ -187,7 +187,7 @@ class DynamicObject::Builder: public kj::DisallowConstCopy {
 public:
   typedef DynamicObject Builds;
 
-  Builder() = delete;
+  Builder() = default;
   inline Builder(decltype(nullptr)) {}
   Builder(Builder&) = default;
   Builder(Builder&&) = default;
@@ -277,7 +277,7 @@ class DynamicStruct::Builder {
 public:
   typedef DynamicStruct Builds;
 
-  Builder() = delete;
+  Builder() = default;
   inline Builder(decltype(nullptr)) {}
 
   template <typename T, typename = kj::EnableIf<kind<FromBuilder<T>>() == Kind::STRUCT>>
@@ -430,7 +430,7 @@ class DynamicList::Builder {
 public:
   typedef DynamicList Builds;
 
-  Builder() = delete;
+  Builder() = default;
   inline Builder(decltype(nullptr)) {}
 
   template <typename T, typename = kj::EnableIf<kind<FromBuilder<T>>() == Kind::LIST>>