Introducing `formulaic`, a high-performance `patsy` "competitor"

Greetings all,

Late last year I had the need to generate sparse model matrices from large pandas DataFrames (dense model matrices would not fit in memory for the dataset I was using). I originally set about trying to patch patsy, but the code was not set up to allow overriding individual methods, and since I felt it would be a didactic experience in any case, I decided to rewrite something like patsy from scratch. The result is Formulaic.

I wasn't expecting much more than the addition of sparse matrix support, but it seems I've also managed to improve the performance of model matrix generation by (in many cases) orders of magnitude, even beating R in many cases. I'm in the process of writing up documentation, and there is some low-hanging fruit in terms of improvements, but I'd love to get some eyes on the project, and would welcome feedback.

The issue links to the Formulaic project and describes it as a rewrite of patsy for sparse model matrices from large pandas DataFrames. Start by reviewing the linked project and the patsy context. No file, test, requested change, or completion condition is specified, so the work is not actionable from this issue alone.