Replaced two-level rule cache with a one-level one.
Replaced two-level rule cache with a one-level one.

This leads to simpler code and also speeds up the benchmark suite execution by
5,89 % on V8.

Detailed results (benchmark suite totals):

---------------------------------
 Test #     Before       After
---------------------------------
      1   24,70 kB/s   26,14 kB/s
      2   24,49 kB/s   26,05 kB/s
      3   24,67 kB/s   25,99 kB/s
      4   24,65 kB/s   26,13 kB/s
      5   24,71 kB/s   26,14 kB/s
---------------------------------
Average   24,64 kB/s   26.10 kB/s
---------------------------------

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pegjs/pegjs,pegjs/pegjs,Sciumo/pegjs,GerHobbelt/pegjs,jlturner/pegjs,Mingun/pegjs,mcanthony/pegjs,wwall/pegjs-fn,GerHobbelt/pegjs,jlturner/pegjs,mcanthony/pegjs,Mingun/pegjs,luisvt/pegjs,luisvt/pegjs,Sciumo/pegjs,crguezl/pegjs,wwall/pegjs-fn,crguezl/pegjs