This replaces the validations on code9 etc. with one continuation that
picks the appropriate parser.

Also relaxes the parser to allow further output codes after the table is
full. Looking at the spec, it seems to me at this times that the
requirement for a clear code when the table is full is a requirement on
producers of PDF files, but not on the file format itself. As far as I
understand, conforming files can be created by a non-conforming process.

Note: The implementation uses a slight trick to handle the last code
(4095) correctly. Quoting the comment in act_output():

    Rather than going through the effort of ensuring that the last
    code is only updated once, we simply assign one more code as a
    dummy.

So, the table is now 4097 entries in actual size. The last one will
receive a bogus update every cycle, so that the last real code does not.
This is less work than actually detecting and avoiding the bogus updates.