Add support for computed property names

[rhendric]

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[rhendric]

After this and #98, the remaining PRs I intend to submit (adding support for rest parameters, default arguments, object/array destructuring in parameters, generators/yield, object literal methods, classes, and the super keyword) start to refactor existing AST constructors, instead of merely adding new ones. If you want to make a backwards-compatible-modulo-exhaustiveness-checks release (to be followed by a backwards-incompatible release shortly thereafter), the time to do that is after this and #98 are merged. If you don't think it matters, I have no issue until waiting until the rest of the work is submitted and merged too.