make constructor of cython HDNode struct more explicit

cython/TrezorCrypto.pyx

@@ -5,12 +5,14 @@ cdef class HDNode:
 
 	cdef c.HDNode node
 
-	def __init__(self, initializer):
-		if isinstance(initializer, HDNode):
-			self.node = (<HDNode>initializer).node
-		elif isinstance(initializer, str) :
-			if c.hdnode_deserialize(initializer, cython.address(self.node)) != 0:
+	def __init__(self, str serialized = None, HDNode copyfrom = None):
+		if copyfrom is not None:
+			self.node = copyfrom.node
+		elif serialized is not None:
+			if c.hdnode_deserialize(serialized, cython.address(self.node)) != 0:
 				raise Exception('Invalid xpub/xprv provided')
+		else:
+			raise Exception('Need to provide serialized or node parameter')
 
 	def xpub(self):
 		cdef char[120] string
@@ -28,11 +30,11 @@ cdef class HDNode:
 		return str(string)
 
 	def public_ckd(self, int i):
-		x = HDNode(self)
+		x = HDNode(copyfrom=self)
 		c.hdnode_public_ckd(cython.address(x.node), i)
 		return x
 
 	def private_ckd(self, int i):
-		x = HDNode(self)
+		x = HDNode(copyfrom=self)
 		c.hdnode_private_ckd(cython.address(x.node), i)
 		return x