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/lib/capnp.pxd

@@ -5,7 +5,7 @@ from capnp.includes cimport schema_cpp
 from capnp.includes.capnp_cpp cimport (
     Schema as C_Schema, StructSchema as C_StructSchema, InterfaceSchema as C_InterfaceSchema,
     EnumSchema as C_EnumSchema, ListSchema as C_ListSchema, DynamicStruct as C_DynamicStruct,
-    DynamicValue as C_DynamicValue, Type as C_Type, DynamicList as C_DynamicList,
+    DynamicValue as C_DynamicValue, Type as C_Type, DynamicList as C_DynamicList, SchemaLoader as C_SchemaLoader,
     SchemaParser as C_SchemaParser, ParsedSchema as C_ParsedSchema, VOID, ArrayPtr, StringPtr,
     String, StringTree, DynamicOrphan as C_DynamicOrphan, AnyPointer as C_DynamicObject,
     DynamicCapability as C_DynamicCapability, Request, Response, RemotePromise, Promise,
@@ -30,6 +30,9 @@ cdef class _StringArrayPtr:
     cdef size_t size
     cdef ArrayPtr[StringPtr] asArrayPtr(self) except +reraise_kj_exception
 
+cdef class SchemaLoader:
+    cdef C_SchemaLoader * thisptr
+
 cdef class SchemaParser:
     cdef C_SchemaParser * thisptr
     cdef public dict modules_by_id