create bytes_read and bytes_written properties

closes #112

Author: Changaco

README.rst

@@ -77,16 +77,15 @@ source code or run ``help(libarchive.entry.ArchiveEntry)`` in a Python shell.
 Displaying progress
 ~~~~~~~~~~~~~~~~~~~
 
-The ``stream_reader`` function and the standard `tell <https://docs.python.org/3/library/io.html#io.IOBase.tell>`_ method can be used to estimate how much of an archive has been read by your program. Here's an example of a progress bar using `tqdm <https://pypi.org/project/tqdm/>`_::
+If your program processes large archives, you can keep track of its progress
+with the ``bytes_read`` attribute. Here's an example of a progress bar using
+`tqdm <https://pypi.org/project/tqdm/>`_::
 
     with tqdm(total=os.stat(archive_path).st_size, unit='bytes') as pbar, \
-         open(archive_path, 'rb') as file, \
-         libarchive.stream_reader(file) as archive:
+         libarchive.file_reader(archive_path) as archive:
         for entry in archive:
             ...
-            pbar.update(file.tell() - pbar.n)
-
-Of course this is only useful if your program processes large archives.
+            pbar.update(archive.bytes_read - pbar.n)
 
 Creating archives
 -----------------

libarchive/ffi.py

@@ -161,6 +161,7 @@ def get_write_filter_function(filter_name):
 
 errno = ffi('errno', [c_archive_p], c_int)
 error_string = ffi('error_string', [c_archive_p], c_char_p)
+ffi('filter_bytes', [c_archive_p, c_int], c_longlong)
 
 # archive_entry
 

libarchive/read.py

@@ -27,6 +27,10 @@ def __iter__(self):
                 return
             yield entry
 
+    @property
+    def bytes_read(self):
+        return ffi.filter_bytes(self._pointer, -1)
+
 
 @contextmanager
 def new_archive_read(format_name='all', filter_name='all', passphrase=None):

libarchive/write.py

@@ -184,6 +184,10 @@ def new_archive_write(format_name, filter_name=None, options='', passphrase=None
         ffi.write_free(archive_p)
         raise
 
+    @property
+    def bytes_written(self):
+        return ffi.filter_bytes(self._pointer, -1)
+
 
 @contextmanager
 def custom_writer(