Add tasks for all the new languages

Task/Power-set/FunL/power-set-1.funl

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+def powerset( s ) = s.subsets().toSet()

Task/Power-set/FunL/power-set-2.funl

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+def
+  powerset( {} ) = {{}}
+  powerset( s ) =
+    acc = powerset( s.tail() )
+    acc + map( x -> {s.head()} + x, acc )

Task/Power-set/FunL/power-set-3.funl

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+import lists.foldr
+
+def powerset( s ) = foldr( \x, acc -> acc + map( a -> {x} + a, acc), {{}}, s )
+
+println( powerset({1, 2, 3, 4}) )