Schema loading from the wire

Cap'n Proto provides a schema loader, which can be used to dynamically
load schemas during runtime. To port this functionality to pycapnp,
a new class is provided `C_SchemaLoader`, which exposes the Cap'n
Proto C++ interface, and `SchemaLoader`, which is part of the pycapnp
library.

The specific use case for this is when a capnp message contains
a Node.Reader: The schema for a yet unseen message can be loaded
dynamically, allowing the future message to be properly processed.

If the message is a struct containing other structs, all the schemas for
every struct must be loaded to correctly parse the message. See
https://github.com/DaneSlattery/capnp_generic_poc for a
proof-of-concept.

Add docs and cleanup

Add more docs

Reduce changes

Fix flake8 formatting

Fix get datatype

test/foo.capnp

@@ -5,3 +5,16 @@ struct Foo {
   name @1 :Text;
 }
 
+
+struct Baz{
+    text @0 :Text;
+    qux @1 :Qux;
+}
+
+struct Qux{
+    id @0 :UInt64;
+}
+
+interface Wrapper  {
+    wrapped @0 (object :AnyPointer);
+}

test/test_load.py

@@ -119,3 +119,30 @@ def test_bundled_import_hook():
     # stream.capnp should be bundled, or provided by the system capnproto
     capnp.add_import_hook()
     import stream_capnp  # noqa: F401
+
+
+def test_load_capnp(foo):
+    # test dynamically loading
+    loader = capnp.SchemaLoader()
+    loader.load(foo.Baz.schema.get_proto())
+    loader.load_dynamic(foo.Qux.schema.get_proto().node)
+
+    schema = loader.get(foo.Baz.schema.get_proto().node.id).as_struct()
+    assert "text" in schema.fieldnames
+    assert "qux" in schema.fieldnames
+    assert schema.fields["qux"].proto.slot.type.which == "struct"
+
+    class Wrapper(foo.Wrapper.Server):
+        def wrapped(self, object, **kwargs):
+            assert isinstance(object, capnp.lib.capnp._DynamicObjectReader)
+            baz_ = object.as_struct(schema)
+            assert baz_.text == "test"
+            assert baz_.qux.id == 2
+
+    # test calling into the wrapper with a Baz message.
+    baz_ = foo.Baz.new_message()
+    baz_.text = "test"
+    baz_.qux.id = 2
+
+    wrapper = foo.Wrapper._new_client(Wrapper())
+    wrapper.wrapped(baz_).wait()