Kill old syntax

Grammar is simplified and parser regenerated.

Changes

* one-line annotations can have a single argument without parentheses
* no more old-style attribute declaration
* no more intern, readable and writable as reserved keyword (they are identifiers now)
* intern, extern and abstract are genuine annotations for AMethPropdef
* no more zoo for AMethPropdef subclasses
* Bonus, introduce the power operation `**` (not implemented yet, but parsed)

TODO: see what is broken in nitpretty and nitx (why this tool need the AST by the way?)