Use faster PyUnicode_FromEncodedObject() for bytes/bytearray.decode().
Add new PyCodec_KnownEncoding() API.
Add new PyUnicode_AsDecodedUnicode() and PyUnicode_AsEncodedUnicode() APIs.
Add missing PyUnicode_AsDecodedObject() to unicodeobject.h
Fix punicode codec to also work on memoryviews.