Collect required lines instead of using them all

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Right now, all the source lines are just put into an array and indexed into when necessary. This of course forces the entire list of lines, which is not the best for performance nor space. Instead, we could collect all the required lines into an IntMap or similar by using the line numbers and styling options from the given Errata.

The makeSourceLines function within the renderSourceLines function contains the logic for which lines are required. It might also be worth investigating refactoring the entire renderSourceLines function so that it isn't so big.