Add some useful _StructSchema, reader, and builder methods

This adds:
 - _StructSchema.fieldnames
 - _DynamicStructBuilder.as_reader
 - _DynamicStruct{Reader,Builder}.schema and __dir__

This makes the reader and builder types much easier to navigate in ipython.

capnp/capnp_cpp.pxd

@@ -33,19 +33,15 @@ cdef extern from "capnp/schema.h" namespace " ::capnp":
         Schema getDependency(uint64_t id) except +
         #InterfaceSchema asInterface() const;
 
-    cdef cppclass MemberForward" ::capnp::StructSchema::Field":
-        pass
-
     cdef cppclass StructSchema(Schema):
-        cppclass FieldList:
-            uint size()
-            MemberForward operator[](uint index)
-
         cppclass Field:
             StructNode.Member.Reader getProto()
             StructSchema getContainingStruct()
             uint getIndex()
-            FieldList getFields()
+
+        cppclass FieldList:
+            uint size()
+            Field operator[](uint index)
 
         Node.Reader getProto()
         FieldList getFields()
@@ -104,6 +100,7 @@ cdef extern from "capnp/dynamic.h" namespace " ::capnp":
             DynamicValueForward.Builder init(char *)
             StructSchema getSchema()
             Maybe[StructSchema.Field] which()
+            DynamicStruct.Reader asReader()
 
 cdef extern from "fixMaybe.h":
     StructSchema.Field fixMaybe(Maybe[StructSchema.Field]) except+