feat(analysis/normed_space/spectrum): Prove the Gelfand-Mazur theorem (#12787)
feat(analysis/normed_space/spectrum): Prove the Gelfand-Mazur theorem (#12787)

**Gelfand-Mazur theorem** For a complex Banach division algebra, the natural `algebra_map ℂ A` is an algebra isomorphism whose inverse is given by selecting the (unique) element of `spectrum ℂ a`.

- [x] depends on: #12132 



Co-authored-by: Eric Wieser <da64e90baec5d084ce3d8e05f3b022ae6d118ff5@gmail.com>

leanprover-community/mathlib,leanprover-community/mathlib