Implement 'in' comparison

This fixes #115.

There was already code in place in `parseCompare` to parse `in` and `not in` as `Compare` nodes. This approach turned out to be problematic because of the implementation of chained `CompareOperand` nodes in `compileCompare`. Since the compilation of `in` and `not in` is not in-order, `compileCompare` would need to be significantly rewritten to support `in`. Such a rewrite could probably include adding better support for chained comparison operators (treating them as a series of ANDed pair comparisons; see https://github.com/jashkenas/coffeescript/issues/78), but this turned out to be complex (especially if we don't want to evaluate some expressions in the comparison chain multiple times), so I decided to consider it out of scope for this PR, and instead implemented `in` as a new `BinOp` node type, which works quite simply.