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

capnp/helpers/deserialize.h

@@ -0,0 +1,12 @@
+#pragma once
+
+#include "capnp/dynamic.h"
+#include "capnp/schema.capnp.h"
+
+/// @brief Convert the dynamic struct to a Node::Reader
+::capnp::schema::Node::Reader toReader(capnp::DynamicStruct::Reader reader)
+{
+    // requires an intermediate step to AnyStruct before going directly to Node::Reader,
+    // since there exists no direct conversion from DynamicStruct::Reader to Node::Reader.
+    return reader.as<capnp::AnyStruct>().as<capnp::schema::Node>();
+}