Add tasks for all the new languages

Task/Fibonacci-sequence/Potion/fibonacci-sequence-1.potion

@@ -0,0 +1,5 @@
+recursive = (n):
+  if (n <= 1): 1. else: recursive (n - 1) + recursive (n - 2)..
+
+n = 40
+("fib(", n, ")= ", recursive (n), "\n") join print

Task/Fibonacci-sequence/Potion/fibonacci-sequence-2.potion

@@ -0,0 +1,11 @@
+iterative = (n) :
+   curr = 0
+   prev = 1
+   tmp = 0
+   n times:
+      tmp = curr
+      curr = curr + prev
+      prev = tmp
+   .
+   curr
+.

Task/Fibonacci-sequence/Potion/fibonacci-sequence-3.potion

@@ -0,0 +1,20 @@
+sqr = (x): x * x.
+
+# Based on the fact that
+# F2n = Fn(2Fn+1 - Fn)
+# F2n+1 = Fn ^2 + Fn+1 ^2
+matrix = (n) :
+   algorithm = (n) :
+      "computes (Fn, Fn+1)"
+      if (n < 2): return ((0, 1), (1, 1)) at(n).
+
+      # n = e + {0, 1}
+      q = algorithm(n / 2)  # q = (Fe/2, Fe/2+1)
+      q = (q(0) * (2 * q(1) - q(0)), sqr(q(0)) + sqr(q(1)))  # q => (Fe, Fe+1)
+      if (n % 2 == 1) :  # q => (Fe+{0, 1}, Fe+1+{0,1}) = (Fn, Fn+1)
+         q = (q(1), q(1) + q(0))
+      .
+      q
+   .
+   algorithm(n)(0)
+.

Task/Fibonacci-sequence/Potion/fibonacci-sequence-4.potion

@@ -0,0 +1,13 @@
+fibonacci = (n) :
+   myFavorite = matrix
+   if (n >= 0) :
+      myFavorite(n)
+   .  else :
+      n = n * -1
+      if (n % 2 == 1) :
+         myFavorite(n)
+      . else :
+         myFavorite(n) * -1
+      .
+   .
+.