Clean generated parser

This series reduce the size of the generated parser.

* First, factorize generated clones (the first commit)
* Second, reduce inlining in the grammar (the other commits)

This was a tidy process because each slight modification in the grammar yields to numerous conflicts often
difficult to understand. But the results worth the pain:

Time of `nitg nitg.nit`
before: 0m10.784s
after: 0m7.956s
gain: 26%

Number of subclasses of `ReduceAction`
before: 780
after: 376
gain: 52%