chore(ring_theory/power_series/well_known): generalize (#5491)
* generalize `power_series.exp`, `power_series.cos`, and `power_series.sin` to a `ℚ`-algebra;
* define `alg_hom.mk'`;
* rename `alg_hom_nat` to `ring_hom.to_nat_alg_hom`;
* drop `alg_hom_int` (was equal to `ring_hom.to_int_alg_hom`);
* add `ring_hom.to_rat_alg_hom`.
