Enable strict concurrency (#159)

### Motivation:

Catch potential data races at build time.

### Modifications:

Enabled strict concurrency complete checking.

### Result:

Fewer potential data races can sneak in.

Package.swift

@@ -45,3 +45,9 @@ let package = Package(
         ),
     ]
 )
+
+for target in package.targets {
+    var settings = target.swiftSettings ?? []
+    settings.append(.enableExperimentalFeature("StrictConcurrency=complete"))
+    target.swiftSettings = settings
+}