chore: shake Std.Time (#6283)

This PR reduces the import closure of `Std.Time` such that it doesn't
have to be rebuilt on every change in `Init.Data`.

Noticed while working on `Init` refactorings.

src/Std/Time/DateTime/Timestamp.lean

@@ -6,6 +6,7 @@ Authors: Sofia Rodrigues
 prelude
 import Std.Time.Internal
 import Init.Data.Int
+import Init.System.IO
 import Std.Time.Time
 import Std.Time.Date
 import Std.Time.Duration

src/Std/Time/Internal/UnitVal.lean

@@ -4,7 +4,6 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Sofia Rodrigues
 -/
 prelude
-import Init.Data
 import Std.Internal.Rat
 
 namespace Std

src/Std/Time/Zoned/Database/Windows.lean

@@ -4,6 +4,7 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Sofia Rodrigues
 -/
 prelude
+import Init.Data.SInt.Basic
 import Std.Time.DateTime
 import Std.Time.Zoned.TimeZone
 import Std.Time.Zoned.ZoneRules