feat(field_theory): more general `algebra _ (algebraic_closure k)` instance (#8658)
feat(field_theory): more general `algebra _ (algebraic_closure k)` instance (#8658)

For example, now we can take a field extension `L / K` and map `x : K` into the algebraic closure of `L`.



Co-authored-by: Anne Baanen <839f20e486a83a313cf46ca4094e2de98129fc5d@users.noreply.github.com>

leanprover-community/mathlib,leanprover-community/mathlib