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);
+}