Here's a new patch incorporating the suggestions I received on python-ideas.

Notable changes are:

  - The name of the methods have been changed to int.tobytes() and
    int.frombytes().
  - The tri-state `little_endian' argument has been removed in favor
    of the `byteorder' argument which takes either the string
    'little' or 'big'.
  - The `byteorder' argument has to be specified explicitly.
  - The variable-length version of int.tobytes() has been removed.
  - The `fixed_length' argument has been renamed to `length'. 
  - The `signed' argument is now keyword-only and now defaults to False.