Add some simple examples of synthesis from examples, with holes

testcases/synthesis/io-examples/Math.scala

+import leon.lang._
+import leon.lang.synthesis._
+
+object Math {
+
+  def fib(a: Int): Int = {
+    require(a >= 0)
+    if (a > 1) {
+      fib(a-1) + fib(a-2)
+    } else {
+      ???[Int]
+    }
+  } ensuring {
+    passes(a, _)(Map(
+      0 -> 0,
+      1 -> 1,
+      2 -> 1,
+      3 -> 2,
+      4 -> 3,
+      5 -> 5,
+      18 -> 2584
+    ))
+  }
+
+  def abs(a: Int): Int = {
+    if (?(a >= 0, a <= 0, a == 0, a != 0)) {
+      a
+    } else {
+      -a
+    }
+  } ensuring { _ >= 0 }
+
+
+
+}