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/serialize.h

@@ -11,4 +11,4 @@ kj::Array< ::capnp::byte> messageToPackedBytes(capnp::MessageBuilder & message,
     kj::ArrayOutputStream out(result.asPtr());
     capnp::writePackedMessage(out, message);
     return heapArray(out.getArray()); // TODO: make this non-copying somehow
-}
+}