Add unused files/deps linter

[LesnyRumcajs]

Issue summary
Given that this repository depends on copy-pasting from builtin-actors, removing some code and adapting it, lots of the code is in danger of ending up as dead code. Such code should not be in the repository.

Let's add some kind of linting to minimize the number of unused code. A sample command for this is described in #175. Better tools may exist for a more complete and accurate linting.

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