[Operators] Make OpeatorTable.standardOperators a stored property

Building standard operator table is not trivial. Make is an immutable
stored property so it stays in memory once it's initialized.

Author: rintaro

Sources/SwiftOperators/OperatorTable+Defaults.swift

@@ -64,7 +64,7 @@ extension OperatorTable {
   /// without requiring access to the standard library source code. However,
   /// because it does not incorporate user-defined operators, it will only
   /// ever be useful for a quick approximation.
-  public static var standardOperators: OperatorTable {
+  public static let standardOperators: OperatorTable = {
     let precedenceGroups: [PrecedenceGroup] = [
       PrecedenceGroup(
         name: "AssignmentPrecedence",
@@ -437,5 +437,5 @@ extension OperatorTable {
       precedenceGroups: precedenceGroups,
       operators: operators
     )
-  }
+  }()
 }