Update to upstream 0.2.3
    
    This is a performance improvement release:
    
    * BitString type implementation rebased onto Python int to behave like
      an integer and improve serialization performance (100x times!).
    * Integer codecs reimplemented to base on Python's built-in integer
      serialization codecs to improve serialization performance (~40%).
    * Decoding performance of Set and Choice types improved by caching
      once computed tags maps.
    * ASN.1 character types refactored to duck-type unicode strings thus
      making them more convenient in text processing context.
    * Documentation updated at many places.
    
    0.2.2
    
    This release fixes a regression in Enumerated ASN.1 type subtyping
    procedure.
    
    Besides that, canonical decoders were hardened to reject non-canonical
    serialization on input.