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.pyx

@@ -362,6 +362,11 @@ cdef class _NodeReader:
     property isEnum:
         def __get__(self):
             return self.thisptr.isEnum()
+    
+    property node:
+        """A property that returns the NodeReader as a DynamicStructReader."""
+        def __get__(self):
+            return _DynamicStructReader()._init(self.thisptr, self)
 
 
 cdef class _NestedNodeReader:
@@ -3231,6 +3236,34 @@ cdef class _StringArrayPtr:
         return ArrayPtr[StringPtr](self.thisptr, self.size)
 
 
+cdef class SchemaLoader:
+    """ Class which can be used to construct Schema objects from schema::Nodes as defined in
+    schema.capnp.
+    
+    This class wraps capnproto/c++/src/capnp/schema-loader.h directly."""
+    def __cinit__(self):
+        self.thisptr = new C_SchemaLoader()
+    
+    def __dealloc__(self):
+        del self.thisptr
+
+    def load(self, _NodeReader reader):
+        """Loads the given schema node.  Validates the node and throws an exception if invalid.  This
+        makes a copy of the schema, so the object passed in can be destroyed after this returns.
+        
+        """
+        return _Schema()._init(self.thisptr.load(reader.thisptr))
+
+    def load_dynamic(self, _DynamicStructReader reader):
+        """Loads the given schema node with self.load, but converts from a _DynamicStructReader
+        first."""
+        return _Schema()._init(self.thisptr.load(helpers.toReader(reader.thisptr)))
+    
+    def get(self, id_):
+        """Gets the schema for the given ID, throwing an exception if it isn't present."""
+        return _Schema()._init(self.thisptr.get(<uint64_t>id_))
+
+
 cdef class SchemaParser:
     """A class for loading Cap'n Proto schema files.