Add rco, a code optimizer for R

[EagleoutIce]

• I have not changed the README.md directly.

rco is a performance optimizer for R programs, which conducts static analysis to identify and apply its optimizations. I am unsure whether the category formatter really fits (it technically reformats the code) but in contrast to the tags I was unable to find any other category let alone documentation for those that would fit (I mean one could argue for linting from a perspective of identifying performance smells, but this seems to be unfitting as well).

[mre]

I considered maintaining a separate list for performance analysis tools (and I have a private repo as well), but the list is quite short so far. If you like, I could invite you and give you write permission to add the tool.
Alternatively, we can add a performance category, but it would take a bit of work to get it on the website.
Open for suggestions.

[EagleoutIce]

I am unsure what the best way to go is here. Personally I prefer a single place to look for such tools but I understand that the separation is fuzzy especially between providing and "simply" using static analyses.
To my knowledge, this is the only "optimizer" for R (if we ignore some alternative interpreters like Ř) so maybe this helps.
But in the end, it may be be better for your to decide what you want this repository to be. If you prefer the separate repository, feel free to invite me and I can add the tool there :)

[mre]

After giving this some more thought, I realized that "performance" is orthogonal to static/dynamic analysis: a tool can aim to improve performance and be a static analysis tool -- even though it's uncommon. That seems to be the case with this, tool, though, so I think it's a good fit for the list. I've modified the code slightly to accept performance as a valid category. Will probably do the same for the dynamic-analysis repo.