Commit 2023-10-12 17:10 bdd2abee

feat: integral of f x • g y over α × β (#7627)

• Generalize integral_prod_mul to integral_prod_smul, add Integrable.prod_smul.
• Rename integrable_prod_mul to Integrable.prod_mul.
• Add integral_fun_fst and integral_fun_snd.