implement buffer protocol (#602)

Fix #479

Author: methane

Diff for: msgpack/_packer.pyx

@@ -44,8 +44,6 @@ cdef extern from "pack.h":
     int msgpack_pack_timestamp(msgpack_packer* x, long long seconds, unsigned long nanoseconds);
     int msgpack_pack_unicode(msgpack_packer* pk, object o, long long limit)
 
-cdef extern from "buff_converter.h":
-    object buff_to_buff(char *, Py_ssize_t)
 
 cdef int DEFAULT_RECURSE_LIMIT=511
 cdef long long ITEM_LIMIT = (2**32)-1
@@ -371,4 +369,10 @@ cdef class Packer(object):
 
     def getbuffer(self):
         """Return view of internal buffer."""
-        return buff_to_buff(self.pk.buf, self.pk.length)
+        return memoryview(self)
+
+    def __getbuffer__(self, Py_buffer *buffer, int flags):
+        PyBuffer_FillInfo(buffer, self, self.pk.buf, self.pk.length, 1, flags)
+
+    def __releasebuffer__(self, Py_buffer *buffer):
+        pass