feat(field_theory/normal): Restriction is surjective (#5960)
feat(field_theory/normal): Restriction is surjective (#5960)

Proves surjectivity of `alg_equiv.restrict_normal_hom`.

Also proves a bijectivity lemma which gives a cleaner construction of `alg_equiv.restrict_normal`.



Co-authored-by: tb65536 <25c750517795e47f172dc8c552ed72c7de260bd4@users.noreply.github.com>

leanprover-community/mathlib,leanprover-community/mathlib