mathlib3

ea772710 - chore(analysis/calculus/{f,}deriv): fix, add missing lemmas (#8574)

chore(analysis/calculus/{f,}deriv): fix, add missing lemmas (#8574) * use `prod.fst` and `prod.snd` in lemmas like `has_fderiv_at_fst`; * add `has_strict_deriv_at.prod`, `has_strict_fderiv_at.comp_has_strict_deriv_at`; * use `set.maps_to` in some lemmas; * golf some proofs.