Better tape parsing mechanisms

[nicb]

Tape parsing mechanism is functional but seriously difficult to handle

• lexing should be subdivided in three different lexers, possibly using rex
• thorough syntax testing should be performed (not only testing with single note.txt texts)
• line numbering should be tested (it's bugged as it is)
• a fifo queue for better error management should be implemented (so that context is handled in a better way)
• parse errors should be handled inside the parsing code (with the yyerror mechanism)

Feel free to open separate issues for each of these items :-)