Expose packed stream with Unpacker.read_bytes()

At present, Unpacker buffers reading from the stream, meaning the stream can no longer be read directly. Unpacker.read_bytes(n) provides access to the underlying data, allowing content of known size to be read without unpacking.

msgpack/_msgpack.pyx

@@ -455,6 +455,18 @@ cdef class Unpacker(object):
             else:
                 raise ValueError("Unpack failed: error = %d" % (ret,))
 
+    def read_bytes(self, Py_ssize_t nbytes):
+        """read a specified number of raw bytes from the stream"""
+        cdef size_t nread
+        ret = ''
+        while len(ret) < nbytes and self.file_like is not None:
+            if self.buf_head == self.buf_tail:
+                self.fill_buffer()
+            nread = min(self.buf_tail - self.buf_head, nbytes - len(ret))
+            ret += PyBytes_FromStringAndSize(self.buf + self.buf_head, nread)
+            self.buf_head += nread
+        return ret
+
     def __iter__(self):
         return self