Add linter

[rohitpaulk]

This PR adds a lint step (using rubocop) to the CI process.

All trivial violations have been fixed, and a few not-so-easy to fix items have been left as pending in rubocop_todo.yml.

The only pending item I see as wildly abnormal is the line length - the high numbers are due to comments such as this one. I couldn't recognize the format used for comments - it had special markers like private:, public:, none that I've seen used in RDoc or YARD markup. If this comment format isn't intentional, I can open another PR to rewrite them in YARD markup.

I've tried to split changes into small commits so that they're easier to review.

[f2prateek]

Awesome, this looks great. Let's merge it to unblock the other PRs.

The only pending item I see as wildly abnormal is the line length - the high numbers are due to comments such as this one. I couldn't recognize the format used for comments - it had special markers like private:, public:, none that I've seen used in RDoc or YARD markup. If this comment format isn't intentional, I can open another PR to rewrite them in YARD markup.

Yeah let's fix those! I don't think it was formatted for a tool, we can definitely switch to using RDoc or YARD here. Is there a reason to prefer one over the other? RDoc does seem more widely used because it's included with Ruby

[f2prateek]

Also as a follow up here - are there any tools that will automate this? e.g. we use https://github.com/prettier/prettier and https://github.com/google/google-java-format in other languages - is there something similar for Ruby?

[f2prateek]

If we can't automate it, a static analysis like this is fine as well - maybe we could add it to the CI to enforce it?