feat: port Data.Finmap (#1591)

Co-authored-by: Arien Malec <arien.malec@gmail.com>
Co-authored-by: qawbecrdtey <qawbecrdtey@naver.com>
Co-authored-by: Komyyy <pol_tta@outlook.jp>

Author: 4 people

Mathlib.lean

@@ -295,6 +295,7 @@ import Mathlib.Data.FinEnum
 import Mathlib.Data.Finite.Basic
 import Mathlib.Data.Finite.Defs
 import Mathlib.Data.Finite.Set
+import Mathlib.Data.Finmap
 import Mathlib.Data.Finset.Basic
 import Mathlib.Data.Finset.Card
 import Mathlib.Data.Finset.Fin