Feat: Port/Data.Seq.Seq (#3187)

Port of `Data.Seq.Seq` following Mathlib3 refactor to avoid `Seq` overload.

Down to two issues:

- [x] `bind_ret` goes like Mathlib3 right up to the last line but doesn't close (I've tried the full Mathlib3 `simp` invocation to no avail.
- [x] `bind_assoc` has issues with single `match` lines gumming up the works, which should `dsimp` away but don't




Co-authored-by: Komyyy <pol_tta@outlook.jp>

Mathlib.lean

@@ -989,6 +989,7 @@ import Mathlib.Data.Real.Sqrt
 import Mathlib.Data.Rel
 import Mathlib.Data.Semiquot
 import Mathlib.Data.Seq.Computation
+import Mathlib.Data.Seq.Seq
 import Mathlib.Data.Set.Accumulate
 import Mathlib.Data.Set.Basic
 import Mathlib.Data.Set.BoolIndicator