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/includes/capnp_cpp.pxd

@@ -446,6 +446,12 @@ cdef extern from "capnp/dynamic.h" namespace " ::capnp":
             DynamicStruct.Pipeline asStruct"releaseAs< ::capnp::DynamicStruct>"()
             Type getType()
 
+cdef extern from "capnp/schema-loader.h" namespace " ::capnp":
+    cdef cppclass SchemaLoader:
+        SchemaLoader()
+        Schema load(Node.Reader reader) except +reraise_kj_exception
+        Schema get(uint64_t id_) except +reraise_kj_exception
+
 cdef extern from "capnp/schema-parser.h" namespace " ::capnp":
     cdef cppclass ParsedSchema(Schema) nogil:
         ParsedSchema getNested(char * name) except +reraise_kj_exception