added fibonacci closure exercise solution

exercises/03-fibonacci/fibonacci.go

@@ -0,0 +1,29 @@
+package main
+
+import "fmt"
+
+// fibonacci is a function that returns
+// a function that returns an int.
+// from cpython/Doc/build/html/library/functools.html
+// [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610]
+func fibonacci() func() int {
+	cur_fib := 0
+	nxt_fib := 0
+	return func() int {
+		if nxt_fib == 0 {
+            nxt_fib = 1
+			return 0
+		}
+		tmp := cur_fib
+		cur_fib = nxt_fib
+		nxt_fib += tmp
+		return cur_fib
+	}
+}
+
+func main() {
+	f := fibonacci()
+	for i := 0; i < 20; i++ {
+		fmt.Println(f())
+	}
+}