feat(data/polynomial): adds map_comp (#3736)
Adds lemma saying that the map of the composition of two polynomials is the composition of the maps, as mentioned [here](https://leanprover.zulipchat.com/#narrow/stream/116395-maths/topic/Polynomial.20composition.20and.20map.20commute).
Co-authored-by: Thomas Browning tb65536@uw.edu
Co-authored-by: Bryan Gin-ge Chen <bryangingechen@gmail.com>
