Add tasks for all the new languages

Task/Arithmetic-Complex/Futhark/arithmetic-complex.futhark

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+type complex = (f64,f64)
+
+fun complexAdd((a,b): complex) ((c,d): complex): complex =
+  (a + c,
+   b + d)
+
+fun complexMult((a,b): complex) ((c,d): complex): complex =
+ (a*c - b * d,
+  a*d + b * c)
+
+fun complexInv((r,i): complex): complex =
+  let denom = r*r + i * i
+  in (r / denom,
+      -i / denom)
+
+fun complexNeg((r,i): complex): complex =
+  (-r, -i)
+
+fun complexConj((r,i): complex): complex =
+  (r, -i)
+
+fun main (o: int) (a: complex) (b: complex): complex =
+  if      o == 0 then complexAdd a b
+  else if o == 1 then complexMult a b
+  else if o == 2 then complexInv a
+  else if o == 3 then complexNeg a
+  else                complexConj a