feat: port FieldTheory.Normal (#4856)

Co-authored-by: Vierkantor <vierkantor@vierkantor.com>
Co-authored-by: Scott Morrison <scott.morrison@anu.edu.au>
Co-authored-by: Thomas Browning <tb65536@uw.edu>
Co-authored-by: Scott Morrison <scott@tqft.net>

Mathlib.lean

@@ -1673,6 +1673,7 @@ import Mathlib.FieldTheory.Laurent
 import Mathlib.FieldTheory.Minpoly.Basic
 import Mathlib.FieldTheory.Minpoly.Field
 import Mathlib.FieldTheory.MvPolynomial
+import Mathlib.FieldTheory.Normal
 import Mathlib.FieldTheory.PerfectClosure
 import Mathlib.FieldTheory.RatFunc
 import Mathlib.FieldTheory.Separable

Mathlib/Data/Polynomial/RingDivision.lean

@@ -589,6 +589,11 @@ theorem isRoot_of_mem_roots (h : a ∈ p.roots) : IsRoot p a :=
   (mem_roots'.1 h).2
 #align polynomial.is_root_of_mem_roots Polynomial.isRoot_of_mem_roots
 
+-- Porting note: added during port.
+lemma mem_roots_iff_aeval_eq_zero (w : p ≠ 0) : x ∈ roots p ↔ aeval x p = 0 := by
+  rw [mem_roots w, IsRoot.def, aeval_def, eval₂_eq_eval_map]
+  simp
+
 theorem card_le_degree_of_subset_roots {p : R[X]} {Z : Finset R} (h : Z.val ⊆ p.roots) :
     Z.card ≤ p.natDegree :=
   (Multiset.card_le_of_le (Finset.val_le_iff_val_subset.2 h)).trans (Polynomial.card_roots' p)