Symbol array literal (%i[] %I[]) syntax support

This code is based on rubinius/rubinius#2269

A few caveats:
* I have no idea what I'm doing. I took existing code by @LTe and messed wtith it until it started working. Please *review it carefully before accepting*.
* I'm not sure if I have to commit generated `grammar.cpp`. It didn't work if I didn't generate it myself.
* There are no tests in this repo so no tests in the pull-request either.