feat: API improvements

Author: maximkrouk

Sources/FunctionalClosures/DataSource.swift

@@ -29,6 +29,10 @@ public class DataSource<Input, Output> {
     public func callAsFunction(_ input: Input) -> Output? {
         projectedValue(input)
     }
+    
+    public func callAsFunction() where Input == Void {
+        projectedValue(())
+    }
 }
 
 
@@ -67,4 +71,8 @@ public class OptionalDataSource<Input, Output> {
     public func callAsFunction(_ input: Input) -> Output? {
         projectedValue?(input)
     }
+    
+    public func callAsFunction() where Input == Void {
+        projectedValue?(())
+    }
 }